feat(app): cite sources in APA, MLA or Chicago, and give studysets a bibliography

A source is now shared by learning paths and studysets (core/citation/):
authors as full names, year, title, what it is a part of, publisher,
ISBN, link, and a citation kept as a file brought it. formatCitation
cites it in APA 7, MLA 9 or Chicago 18: it inverts and abbreviates
names itself (particles, suffixes, names written as already cited stay),
sets a work of its own in italics and a part of something apart from
the whole, and writes a missing year as "n.d." in APA and Chicago.

- Settings: a Citations card per device, with a preview and a link to
  the style's own guide.
- Studysets: a Bibliography button opens the works cited, to read and,
  behind its pencil, to change (studyset schema 4).
- Steps: sources edited through the same sheet, opened from an
  "N sources" button in the runner (step schema 2; schema 1 reads on).
- Path import: steps' sources, text kept as written, the old format's
  "citation" read as what a source is part of.

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

@@ -83,7 +83,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 - **Manual dependency injection.** One `Dependencies` object (`lib/src/core/dependencies/`) is the composition root, built at startup (`Dependencies.create()`) and exposed through the `DependenciesScope` InheritedWidget. It hands features repositories only. No service locator, no DI framework, no globals (the scaffold's global `client` moved into it). Why: every dependency is a typed, greppable field, and nothing is resolved by magic.
 - **One graph per context.** The dependency graph belongs to one instance and one signed-in person; when either changes, a `KeyedSubtree` disposes the whole subtree and rebuilds it. Why: nothing from one person's session (blocs, streams, caches) can leak into another's.
   - **App-wide vs. per person.** `Dependencies` holds what outlives a person (the client, auth). Everything of the signed-in person (their local database, its sync, their repositories) is `PersonDependencies`, opened by `PersonScope` inside `SessionGate`'s keyed subtree (`Dependencies.openPerson`) and disposed with it; widgets reach it through `PersonScope.of(context)`.
-  - **The device's own settings** (appearance, language; `core/device_settings/`) belong to the device, not to a person: kept in `<app support>/device_settings.json`, read in `Dependencies.create()` so the first frame already has them, and provided by `DeviceSettingsScope` above `SessionGate`'s keyed subtree, which both its plain app and `PersonApp` read. What should follow the person to their other devices goes into their synced `PersonSettings` instead.
+  - **The device's own settings** (appearance, language, citation style; `core/device_settings/`) belong to the device, not to a person: kept in `<app support>/device_settings.json`, read in `Dependencies.create()` so the first frame already has them, and provided by `DeviceSettingsScope` above `SessionGate`'s keyed subtree, which both its plain app and `PersonApp` read. What should follow the person to their other devices goes into their synced `PersonSettings` instead.
   - **Every person has an app of their own** (`PersonApp`, `lib/src/app/`), which `SessionGate` builds inside the keyed subtree: a `MaterialApp.router` with its own `GoRouter`, and `PersonScope` around the router's navigator (`MaterialApp.builder`). So every screen and dialog is inside the person's scope and reads their repositories from `PersonScope.of(context)` wherever it is built, and a change of person disposes the router with every screen on it and starts again at home. Until someone is signed in, `SessionGate` shows a plain `MaterialApp`: the spinner, or the failure with a way to try again.
 - **Platform plugins sit behind small interfaces in `Dependencies`**, with fakes in tests: picking files (`FilePicker`, `file_selector`), playing sounds (`SoundPlayer`, `audioplayers`), playing YouTube videos (`YoutubeVideos`, `youtube_player_iframe` where a web view exists, else the browser), reading PDFs (`PdfEngine`, pdfrx on PDFium, which `flutter test` can't load: widget tests use `FakePdfEngine`, whose PDF bytes say their pages and chapters), reading Anki packages on another isolate (`AnkiPackageReader`; a widget test's fake clock can't wait for one), and the Pomodoro timer's side of the system (`PomodoroPlatform`, `FakePomodoroPlatform` in tests).
 - **Material's `Card` widget clashes with the domain's `Card`**: files that need both import `material_ui` with `hide Card`.
@@ -144,6 +144,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
   - **Files** (ADR 0004, rule 5): `AssetEndpoint` (`requireLogin`) takes uploads into the caller's own space by SHA-256 (`startUpload` gives Serverpod's upload description, or null when the file is there already and claims it anew; `finishUpload` keeps it only if its content has that hash) and serves an asset row's file to its space's owner and members only, `NotFoundException` otherwise, also for a deleted row (the sync engine hides tombstones). Files live in the private database storage at `spaces/<space>/<sha256>`, at most 20 MB each, and within limits (`file_limits.dart`, from the environment: an anonymous person 100 MB, an identified one 500 MB or what a `file_allowance` row grants them, the instance 15 GB), checked when an upload starts (unfinished uploads count, or they could fill the disk unnoticed) and again when it finishes (only what is kept counts), refused with `FileLimitExceededException`; `deletePerson` deletes the person's files and allowance, and `FilePruningFutureCall` deletes daily those no live row refers to (or never finished), 7 days after they were last claimed. In the app, `AssetRepository` (`core/asset/`) keeps files by hash in the person's folder (`files/`, uploads waiting marked in `files/uploads/`), uploads after each sync that went through, and downloads on demand, checking the hash; every hash is checked to be one before it becomes part of a path. Content names a file by the URL `asset:<asset row id>` (`assetUrl`, `RichContent.assetIds`): images are the editor's `image` block, sounds an `audio` block (`core/editor/file_blocks.dart`), and any other URL is never fetched. `StoredFileImage` loads through the repository like Flutter's `NetworkImage`, keyed by the person's repository too, so no image crosses people.
   - **PDFs** (`feature/pdf_source/`): a `pdf_source` row per PDF in a studyset (payload: title, file name, the asset of its file, page count, reading deadline; last writer wins) and an append-only `pdf_reading` log (`reached` with a page, `reset`): the furthest page is the furthest reached since the last reset, so devices never conflict. A PDF's file is an asset of up to 50 MB (other files 20 MB): the server takes up to 50 MB and deletes a stored file over 20 MB that doesn't start with `%PDF-`. Deleting a PDF deletes its asset row too unless another PDF has the same file (asset rows are otherwise never deleted). Cards and notes tied to a page keep it in their payload (`source`, `core/content/pdf_page_ref.dart`; card schema 4, note schema 2); a screenshot is an `image` block with `pdf` and `page` attributes.
   - **The Pomodoro timer** (`feature/pomodoro/`): the profiles follow the person (`PersonSettings.pomodoro`, settings schema 4), the running timer is the device's own (`PomodoroRun` in `<person folder>/pomodoro.json`, `PomodoroRunStore`). `PomodoroRun` is a pure state machine on the time: a phase that ends leads to the next, running if its profile starts it by itself, else paused at its full length; a timer kept while the app was closed catches up (`caughtUpTo`). `PomodoroBloc` sits above the router (`PersonApp`), and `PomodoroPresenter` hands every change to `PomodoroPlatform` as a `PomodoroNotice` in the app's language, and its buttons back. **Android:** the app's own Kotlin (`PomodoroService.kt`; MethodChannel `app.halkyon.learn/pomodoro`, EventChannel `…/buttons`): a `specialUse` foreground service with a countdown notification (chronometer), Pause/Resume, Skip, Stop, on Android 16 a Live Update (`NotificationCompat.ProgressStyle`, the cycle's phases as segments, `setRequestPromotedOngoing`); an exact alarm (`USE_EXACT_ALARM`) alerts at the phase's end with the app closed; Do Not Disturb through the interruption filter, the one before kept in shared preferences and restored. With the app swiped away its buttons open it (a service may not start an activity from a notification's button since Android 12, so the notification is posted again with activity intents), and Stop works alone, leaving a mark the app takes at its next start (`takeStoppedOutside`) to forget its timer; a profile's next phase only starts by itself while the app runs. Dart's timers sleep with the device, so `PomodoroBloc` also looks every 15 s while a phase runs, and the alarm's receiver wakes it (`phaseEnds`); a kept timer is caught up before it first shows, or its ended phase would alert again. No notification plugin: flutter_local_notifications had Live Updates only in a prerelease needing compileSdk 37 (checked 2026-10-07). **Linux:** `LinuxPomodoroPlatform`, over package:dbus: where the top bar shows icons (GNOME with the AppIndicator extension, built into Ubuntu; KDE) the timer counts down there with its buttons in the icon's menu (`StatusNotifierItem`, written in Dart and laid out as libayatana-appindicator lays itself out, within the snap's rules, `desktop-legacy` of snapd v2.77: the item at `/org/ayatana/NotificationItem/<id>`, the only path the label's signal `XAyatanaNewLabel` may come from, registered with the watcher by that path, since a snap may bind no `org.freedesktop.StatusNotifierItem-*` name; the menu at `/MenuBar`; registered again when the watcher restarts); elsewhere the timer shows in the app alone, and on GNOME the Pomodoro card suggests the extension until dismissed, asking GNOME Shell to install or switch it on (`InstallRemoteExtension`, `EnableExtension`) or opening its page (decided 2026-10-07: no notification stands in; xdg-desktop-portal's Background status is for Flatpaks only). An alert with sound comes through `org.freedesktop.Notifications` when a phase ends while the app runs, and GNOME's `show-banners` goes off while focusing, through `gsettings`; the value before and the dismissed suggestion are kept in `<app support>/`. Calls to the bus go one at a time (`SerialQueue`).
+  - **Citations** (`core/citation/`): a `Source` is plain text parts as the person typed them (authors as full names, one per line; year; title; `container`, what it is a part of; publisher; ISBN; link; `raw`, a citation a file brought, shown while nothing else is cited), and `formatCitation` cites it in the device's `CitationStyle` (APA 7, MLA 9, Chicago 18 notes and bibliography, decided 2026-10-07): it splits and abbreviates the names itself (inverted names, particles, suffixes; names written with `&` or "et al." stay as written), sets a work of its own in italics and a part of something apart from the whole by `container`, and writes a missing year as "n.d." in APA and Chicago. A step's sources (step schema 2) and a studyset's bibliography (`bibliography`, studyset schema 4, last writer wins like the rest of its payload) are lists of them, shown and changed through `SourceList` and `showSourcesSheet`, in reference-list order (alphabetical, whatever order they were added in); the path import reads them too, text as `raw` and the old format's `citation` as `container`.
   - **Imported cards** say where they came from in their first revision's payload (`origin`, card schema 3, e.g. `anki`); the dashboard doesn't count them as written. The Anki import (`feature/anki/`) reads `.apkg`/`.colpkg` packages of every format on an isolate of its own (`AnkiPackageReader`), zstd through our fork of `pure_zstd` (Anki's frames don't record their size), and its fixtures come from Anki itself (`tool/anki/make_fixtures.py`).
   - **Every client migration keeps a device's rows** (`test/core/sync/device_migration_test.dart` replays each on a database with a row in every synced table of ours; SQLite rebuilds tables to add foreign keys, which Serverpod's runner does with foreign keys off). A new synced table needs a row in that test, which fails until it has one.
   - **Every synced table indexes `spaceId`** (`indexes: <table>_space_idx: fields: spaceId`, e.g. `studyset.spy.yaml`): deleting a space cascades into every synced table, and Postgres doesn't index foreign keys by itself, so account deletion and pruning would otherwise read whole tables.