feat(studyset): an empty tab says its material is in the topics, and leads there

People testing the app looked for the material of a studyset that groups
topics in its own empty tabs, which said "No flashcards yet" and how to
add some. A tab with none of its own while the topics hold some now says
so: "The flashcards are in its topics", each topic holding some with how
much (everything inside it counted), opening it; for flashcards Browse
all of them in one list; and, quieter, how to add some here after all.
Opened for the first time, such a studyset shows the first tab its topics
fill. The tabs' counts stay the studyset's own.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

AGENTS.md

@@ -100,7 +100,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
   - **Routes carry ids, never objects.** A screen finds its item in the repository's live list with `FoundById` (`core/widget/`): a spinner until the list arrives, then the item, or a screen saying it no longer exists (deleted on another device, or an old link). A screen that deletes its item closes, and keeps showing the item while it goes. Why: every location can be built from its path alone, and no screen goes on showing something that has gone.
 - **The look is one theme** (`core/theme/`): `appTheme(brightness)` with hand-written light and dark colour schemes around the brand's cobalt and copper; `Theme.of(context).extras` (`ThemeExtras`) for the colours Material has no role for (states, the four FSRS ratings, a starred star); `PaletteColor` for the 20 colours a person can give content; snack bars floating above the navigation bar as Material 3 places them, in its inverse colours (dark in the light theme), lined up with the floating bar, 560 wide and centred from medium windows on (`FittedSnackBars`, decided 2026-10-12; Flutter's default is Material 2's fixed bar across the foot); `TopGradient` for a colour washing down from the top of every tab's first screen, Settings and Connect Claude (cobalt) and of every screen of a studyset (its colour, own or inherited, which `_inStudyset` in the router hands down as `ScreenColor`): the studyset, studying it, its cards, writing them, its notes, paths and dictionary; behind a clear app bar, in dark mode a deep shade (42 % of the colour over black), in light mode a tint, held to WCAG AA for the theme's text. From medium windows on, a `TintFrame` takes the colour of the screen in view (`ScreenTint`, claimed by the `TopGradient` whose tab shows and whose route is on top), fading out by 60 % of the height: the rail sits on it, transparent, and the screen is a rounded panel inset beside it, its own wash half as strong; screens over the tabs (studying, the card browser and composer, Settings) sit in the same frame without the rail (`FramedScreen`). As Vaia does, decided 2026-10-10. The PDF reader and the reading timeline stay plain. `theme_test.dart` holds text to WCAG AA contrast (4.5:1) on each palette colour and on the dark theme's primary colour and surface. **Fonts are bundled, never fetched** (`core/theme/app_fonts.dart`, `assets/fonts/ATTRIBUTION.md`): Noto Sans is the app's font on every device (Latin, Greek with polytonic, Cyrillic); Noto Sans JP, SC, TC and KR fill in Japanese, Chinese and Korean, Japanese first unless a studyset's language puts its own first (`StudysetLanguageScope`, which every studyset route wraps its screen in through `_inStudyset`, and the library's and Explore's cards each around their studyset; Han characters take the first listed font's shapes); each is one variable font holding every weight, and their OFL licences join the licence list. Scripts none covers (Arabic, Hebrew, Thai, …) fall back to the device's own fonts. A studyset's languages (`frontLanguage`, `backLanguage`, BCP 47) are given together or inherited together from the nearest studyset above (`languagesOf`). Never add `google_fonts` or anything else that downloads fonts. Layouts follow Material 3's window-size classes (`context.windowSize`, `core/layout/`), and running text stays within `readableContentMaxWidth`. Icons are Lucide (`lucide_icons_flutter`), except a starred star and the marks of a choice not yet checked (filled box and circle), which need the fill Lucide lacks; the Material icons left in older screens go when those are restyled. Shared widgets are in `core/widget/` (`SectionCard`, `AppLoadingIndicator` (every spinner is Material 3 Expressive's, in buttons `ButtonLoadingIndicator`, pulling a list down `AppRefreshIndicator`, never Flutter's older circular one), `confirmDestructiveAction`, `FoundById`, `TitleWithKicker`, `ContentCountsLine`, `showUndoSnackBar`, `ScrollHidingFab`, `PaletteColorPicker`, `CountStepper`, `requiredDecoration` (every required field: while empty its outline stronger, in the primary colour, and "Required" below it; never an asterisk) and `FieldInfoButton`, `ReorderToggle`, `ReorderGrip`, `CollapsingRow`, `CountBadge`, `liftWhileDragged`, `LaunchScreen` (what shows while the app opens: the kingfisher, "Halkyon" over "Learn"), `SliverCentered` and `EmptyListMessage` (what an empty list shows: icon, "No … yet", and which menus add some), and the sheets below).
 - **A studyset's options are the same wherever a menu of it opens** (`studysetOptions` and `doStudysetOption` in `studyset_actions.dart`, decided 2026-10-12): its card in the library, its own screen, a topic beside it. Import content runs where the studyset's blocs are, on its screen: from elsewhere it opens the screen, which asks what to import as it opens (`?import=content`).
-- **A studyset's content is in three parts** (`feature/studyset/widget/studyset_parts.dart`, decided 2026-10-12, as Vaia has it): Material (its learning paths, flashcards, notes and files in tabs, the paths first as the way through the rest (decided 2026-10-13: a path is material too, which the plan refers to), each saying how many it holds, an empty one nothing so the four fit a phone more often, each list whole with its tools at the end of the tabs' row, which fold one by one where room runs short rather than cutting the tabs off (`CollapsingRow`: Browse's words first, then the filter into the ⋮ menu, then Browse; as the studyset's own actions fold into icons); decided 2026-10-12 after a trial of sections one below the other, the first three of each behind Show all, as Vaia stacks them: the lists run to hundreds of cards, and tabs are Material 3's way within a part; the tab is kept as the studyset's part, while the material shows, and opened for the first time it opens on the first tab with something in it), Progress (always, nothing yet where nothing was studied, so the switch never changes as reviews are read, decided 2026-10-12: how much is learnt, what is due today, the study time this week from the reviews' durations, each at most `Review.maxStudyTime`, `StudysetActivityCubit`) and Study plan (the paths finished; Next up: the cards due, then the next step of each path under way, or the first path not finished when none is; how far each path has come, its steps done in a bar, each opening the path; managing the paths is the material's; later an exam's date and its schedule, `PathListState.stepsDone`). `StudysetPartSwitch` chooses between them in the header: below expanded windows in a line of its own under the title, from there on in the path's line, which the topics leave free there, centred above the material's column (at the line's end where that column is narrower than the switch; the path folds before it); it ticks as the navigation bar does. **A studyset opens on the part it was left on, on this device** (`StudysetTabs`, `studyset_tabs.json` in the person's folder, read as their data opens, decided 2026-10-11; a tab kept before the parts counts as its part); opened there for the first time, on the study plan while a path is unfinished, else on the material, showing that it loads until it knows rather than one part and then another. Material added or imported shows in its part (`ContentTabCubit`).
+- **A studyset's content is in three parts** (`feature/studyset/widget/studyset_parts.dart`, decided 2026-10-12, as Vaia has it): Material (its learning paths, flashcards, notes and files in tabs, the paths first as the way through the rest (decided 2026-10-13: a path is material too, which the plan refers to), each saying how many it holds, an empty one nothing so the four fit a phone more often, each list whole with its tools at the end of the tabs' row, which fold one by one where room runs short rather than cutting the tabs off (`CollapsingRow`: Browse's words first, then the filter into the ⋮ menu, then Browse; as the studyset's own actions fold into icons); decided 2026-10-12 after a trial of sections one below the other, the first three of each behind Show all, as Vaia stacks them: the lists run to hundreds of cards, and tabs are Material 3's way within a part; the tab is kept as the studyset's part, while the material shows, and opened for the first time it opens on the first tab with something in it; a tab with none of its own while the studyset's topics hold some says that it is in them, each topic holding some with how much, everything inside it counted, opening it, Browse all for flashcards, and how to add some here after all, decided 2026-10-14 after people testing the app looked for a grouping studyset's material in its empty tabs; the tabs' counts stay its own), Progress (always, nothing yet where nothing was studied, so the switch never changes as reviews are read, decided 2026-10-12: how much is learnt, what is due today, the study time this week from the reviews' durations, each at most `Review.maxStudyTime`, `StudysetActivityCubit`) and Study plan (the paths finished; Next up: the cards due, then the next step of each path under way, or the first path not finished when none is; how far each path has come, its steps done in a bar, each opening the path; managing the paths is the material's; later an exam's date and its schedule, `PathListState.stepsDone`). `StudysetPartSwitch` chooses between them in the header: below expanded windows in a line of its own under the title, from there on in the path's line, which the topics leave free there, centred above the material's column (at the line's end where that column is narrower than the switch; the path folds before it); it ticks as the navigation bar does. **A studyset opens on the part it was left on, on this device** (`StudysetTabs`, `studyset_tabs.json` in the person's folder, read as their data opens, decided 2026-10-11; a tab kept before the parts counts as its part); opened there for the first time, on the study plan while a path is unfinished, else on the material, showing that it loads until it knows rather than one part and then another. Material added or imported shows in its part (`ContentTabCubit`).
 - **Statistics** (`feature/statistics/`, decided 2026-10-12, as Vaia has them): the Profile tab's second tab, for anonymous people too. The time studied in all, per day (every day from the first studied to today counted) and the most in one day, and by days, weeks or months (ten, ten, twelve bars) the study time and the ratings, as bars the app draws itself (`BarChart`, no chart package); a tap on a bar says what it holds. Computed on the device from every review (`studyStatistics`, `StatisticsCubit`), every rating counted, crammed ones too, a fresh start not, and **a review counts as at most a minute of study** (`Review.maxStudyTime`, as Anki does by default: a card left on the screen over lunch isn't an hour of study; stored durations stay as they were), here and in a studyset's Progress. What waits in Recently deleted counts nowhere, here or on Home.
 - **Lists that can grow long are built as they scroll into view**: a studyset's parts are `CustomScrollView`s holding each list's slivers (`SliverList`, `SliverReorderableList`), and what a list says in place of its rows sits in a `SliverCentered`. Never a `shrinkWrap` list inside another scrolling view: that builds every row at once, and 1,000 cards then froze the window for seconds, long enough for the desktop to offer to kill the app. Rows are put in another order in a Reorder mode (`ReorderToggle` beside the list; a `ReorderGrip` in place of each row's options; positions from `core/sync/fractional_position.dart`), never with Flutter's default drag handles, which on the desktop lie over a row's trailing end, its … button. Flashcards have no Reorder (decided 2026-10-08): they keep the order they were created or imported in, which new cards and cramming follow, and sub-studysets put lessons in order; no one drags a card among a thousand. **The card browser** (`feature/card/widget/card_browser_screen.dart`, decided 2026-10-09: the older design, gaps filled) opens from Browse beside the flashcards' tab, over the tabs (`/studysets/:id/flashcards`, `?subtree=1` with everything inside, which a line below the title says; the title has how many cards beside it): grid or detail (one card beside the list, which follows it; on a phone above a strip), as last left per studyset on the device (`DeviceSettings.detailedBrowsers`); search over front, back, reading, meanings, hints, explanation and tag names (katakana as hiragana, romaji finding kana, accents, tone marks and umlauts left out, so strasse finds Straße and xue xi xué xí; `card_browser.dart`); sort (own order, newest, oldest, A–Z by reading, by bare letters, due first); a filter sheet (kind, one tag, state: new, learning, due, buried, reported); and choosing cards (a toggle; shift-click or long-press for a range) to move (`showCardMoveSheet`), tag, untag, start afresh or delete with Undo. It reads every card of the studysets, as lists do (schedules from the review repository's cache).
 - **Forms and pickers open as sheets**: `showAdaptiveSheet` with a `SheetScaffold` (title and close button on top, the body scrolling with a scrollbar, always shown on the desktop, and Material 3's lines under the title and above the footer while some of the body lies beyond them, the action pinned below) shows a bottom sheet below expanded windows and a dialog from there on. A sheet that asks before discarding changes (a `PopScope`) passes `swipeable: false`: a bottom sheet swiped away closes without asking its `PopScope` (Flutter issue 152120, open with v3.47). A sheet offering an Undo passes `hasSnackBars: true`, so its snack bars show on it rather than behind its barrier. A sheet running something that mustn't be cut off (an import) is `busy` meanwhile: its close button is greyed and back does nothing; it is shown with `swipeable: false`, since a swipe closes a bottom sheet regardless. A screen whose blocs import (a studyset's PDFs and paths) greys its back button and path likewise, as leaving would close them; an import of paths shows the study plan at once, how far it has come above the paths (`PathImportProgress`). A picker that changes something outside it reports every change as it happens (`onChanged`) rather than as its result, since a sheet swiped away has none. What needs the whole window to be written or picked (a studyset's description, a step's feedback, a map question's region: `editRichText`, `pickRegion`, decided 2026-10-12) is a field in the form (`ChevronField`: the value, a chevron) opening a full-window route with Save in its app bar, where nothing scrolls around a map, so the wheel alone zooms it; a country's own map (Japan's prefectures) is offered there only once the country, or a region within it, is picked.