feat(editor): links, and copying and pasting with every format

Text gets a link from the format bar's new link button or with Ctrl+K
(⌘K): the address for the selection, an empty one taking it away, or at
a caret the text as well, written there linked. An address without its
scheme leads to the web. This replaces the editor's own link menu, which
had no button and spoke English only.

Copying and pasting between the app's editors keeps every format: bold,
links, furigana, formulas, callouts, spoilers. The system's clipboard
holds plain text only (the editor's own copy drops the HTML it makes),
so the blocks copied last stay in memory beside it, and a paste takes
them while the clipboard still holds their text; anything else pastes as
plain text, as before, and other apps get plain text. On phones, which
had no way to copy at all, a bar over selected text copies, cuts, pastes
and selects all (the editor's MobileFloatingToolbar), never over a block
taken whole, whose own buttons a tap is for.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMm7usAo6q9ZZRoPkWjoxi
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

AGENTS.md

@@ -89,7 +89,7 @@ A spaced-repetition learning app: studysets of flashcards, reviewed on an FSRS s
 - **Material's `Card` widget clashes with the domain's `Card`**: files that need both import `material_ui` with `hide Card`.
 - **State: `flutter_bloc`, uniformly.** Blocs fold repository streams into immutable states with value equality, are provided as close to their consumer as possible, and log through one global `BlocObserver`. Why: the data layer is stream-shaped, event → state machines test without widgets, and one pattern everywhere is itself the point.
 - **Value equality comes from `equatable`**, mixed in (`with Equatable`), for models and states alike; `leancode_lint`'s `missing_equatable_props` catches a field left out of `props`.
-- **Rich text is `RichContent`** (`lib/src/core/content/`): the editor's document JSON plus a `formatVersion`, pure Dart, so models and the server can use it. `lib/src/core/editor/` converts it to and from the editor's `Document` (giving tables the decimal sizes the editor casts to: JSON from imports and archives may hold whole numbers, or none; our fork of the editor fits a read-only table's rows to their cells, which upstream's leaves at their stored height) and holds the one editing widget (`RichContentEditor`) and the one read-only renderer (`RichContentView`).
+- **Rich text is `RichContent`** (`lib/src/core/content/`): the editor's document JSON plus a `formatVersion`, pure Dart, so models and the server can use it. `lib/src/core/editor/` converts it to and from the editor's `Document` (giving tables the decimal sizes the editor casts to: JSON from imports and archives may hold whole numbers, or none; our fork of the editor fits a read-only table's rows to their cells, which upstream's leaves at their stored height) and holds the one editing widget (`RichContentEditor`) and the one read-only renderer (`RichContentView`). Copying and pasting keep every format between the app's editors (`EditorClipboard`, decided 2026-10-11): the system's clipboard holds plain text only, so the blocks copied last stay in memory beside it and a paste takes them while it still holds their text, plain text otherwise; the editor's own copy, cut and paste are replaced (Ctrl+C, X, V), and on phones a bar over selected text (the editor's `MobileFloatingToolbar`, over text only) copies, cuts, pastes and selects all. Links come from the format bar's link button and Ctrl+K (`link.dart`, in place of the editor's own link menu, English only); an address without its scheme leads to the web.
   - **Blocks** beyond the editor's own (callout, spoiler, formula, image, sound, YouTube video, study button) keep their type and attributes as named in `core/content/content_blocks.dart`, which `RichContent` reads too (their text counts in `plainText`, and a video or study button isn't blank). An `EditorBlocks` lists how each type is built, which can be inserted (the format bar's Insert sheet and the desktop's `/` menu show the same list) and its typing rules; the core's are `coreBlocks`, and a feature's (the study button, `feature/study/`) reach the editors through `EditorBlocksScope`, which `PersonApp` fills, since the core can't know features. One builder serves editor and view: a block asks `editorState.editable`, and read-only shows what it is for (a spoiler hidden until revealed, a formula set, a video's play). A block taken whole mixes in `AtomicBlock` (selection, the remove button); unknown types show "needs a newer version" and stay as they are. New block types keep `formatVersion` 1: raising it would hide whole studysets and paths in older apps.
 - **Time comes from `clock.now()`** (`package:clock`), never `DateTime.now()`, in server and app. Why: scheduling, streaks and expiry are deterministic under test (`withClock`, `fakeAsync`), and because Serverpod itself reads `clock.now()` (e.g. session expiry), `withClock` controls its behaviour too.
 - **UTC for storing and sending, local time for display and day boundaries.** Serverpod sends `DateTime` as UTC and stores it in `timestamp without time zone` columns. Compare and compute in UTC; call `.toLocal()` only to display. "Due today", streaks and daily goals use the learner's **local** calendar day, computed on the client; the server never needs day boundaries.