feat(app): notes in the studyset screen, written in the whole window

- A Notes tab beside Flashcards lists a studyset's notes in order: the
  title ("Untitled note" without one) and when each was last edited
  ("Edited 5 minutes ago", by the calendar from yesterday on, the date
  after a week); Edit and Delete behind its options. Deleting asks,
  then offers Undo for a moment (`NoteDeletions`). With none, the tab
  says so.
- "Create a note" in Add material opens a new note over the tabs; so
  does a note of the list. The note: a title, its tags once it exists,
  and its text at reading width; the app bar says how saving stands.
- `NoteEditorBloc` saves after 5 seconds without typing, when the app
  goes to the background, and when the note is closed (the bloc saves
  as it closes, so leaving never waits, and the write outlives the
  screen). Each save builds on the revisions the editor started from;
  another device's change takes over a note with nothing unsaved, and
  merges with one that has. A note changed differently on two devices
  is read-only until the person keeps a version of each field in
  conflict. A new note is created by its first save, never when left
  blank; a note deleted elsewhere says it no longer exists.
- The editor gains a format bar, docked below it while it has the
  focus (`RichContentEditor(formatBar: true)`): alignment, bold,
  italic, underline, superscript and subscript. It takes no focus, so
  the selection stays. Superscript and subscript are inline marks of
  their own, drawn by the font's raised and lowered glyphs in editors
  and read-only text alike.
- The library's and the topics' counts include notes (a studyset with
  what is inside it, as for cards).

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

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -158,6 +158,17 @@ class AppLocalizationsEn extends AppLocalizations {
     return '$_temp0';
   }
 
+  @override
+  String studysetCountNotes(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count notes',
+      one: '1 note',
+    );
+    return '$_temp0';
+  }
+
   @override
   String get studysetNoContent => 'No content yet';
 
@@ -200,6 +211,128 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get studysetTabFlashcards => 'Flashcards';
 
+  @override
+  String get studysetTabNotes => 'Notes';
+
+  @override
+  String get studysetCreateNote => 'Create a note';
+
+  @override
+  String get notesEmpty => 'No notes yet';
+
+  @override
+  String get notesEmptyHint => 'Notes will appear here when you add them.';
+
+  @override
+  String get noteUntitled => 'Untitled note';
+
+  @override
+  String noteEdited(String when) {
+    return 'Edited $when';
+  }
+
+  @override
+  String get timeJustNow => 'just now';
+
+  @override
+  String timeMinutesAgo(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count minutes ago',
+      one: '1 minute ago',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String timeHoursAgo(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count hours ago',
+      one: '1 hour ago',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String timeDaysAgo(int count) {
+    String _temp0 = intl.Intl.pluralLogic(
+      count,
+      locale: localeName,
+      other: '$count days ago',
+      one: 'yesterday',
+    );
+    return '$_temp0';
+  }
+
+  @override
+  String get noteOptions => 'Note options';
+
+  @override
+  String get noteEdit => 'Edit';
+
+  @override
+  String get noteDelete => 'Delete';
+
+  @override
+  String get noteDeleteTitle => 'Delete note?';
+
+  @override
+  String get noteDeleteMessage =>
+      'This note will be removed from the studyset. You can undo this for a few seconds.';
+
+  @override
+  String get noteDeleteConfirm => 'Delete';
+
+  @override
+  String get noteDeleted => 'Note deleted';
+
+  @override
+  String noteNewTitleFor(String title) {
+    return 'New note in \"$title\"';
+  }
+
+  @override
+  String noteTitleFor(String title) {
+    return 'Note in \"$title\"';
+  }
+
+  @override
+  String get noteTitleHint => 'Title';
+
+  @override
+  String get noteEditing => 'Editing…';
+
+  @override
+  String get noteSaving => 'Saving…';
+
+  @override
+  String get noteSaved => 'Saved';
+
+  @override
+  String get noteSaveFailed => 'Save failed';
+
+  @override
+  String get noteConflict =>
+      'This note was changed differently on another device. Keep one version to go on writing.';
+
+  @override
+  String get noteFieldTitle => 'Title';
+
+  @override
+  String get noteFieldBody => 'Text';
+
+  @override
+  String get noteKeepThis => 'Keep this';
+
+  @override
+  String get noteAddTags => 'Add tags';
+
+  @override
+  String get noteTags => 'Tags';
+
   @override
   String get studysetFlashcardsEmpty => 'No flashcards yet';
 
@@ -692,6 +825,30 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get composerKeepThis => 'Keep this';
 
+  @override
+  String get formatAlignLeft => 'Align left';
+
+  @override
+  String get formatAlignCenter => 'Center';
+
+  @override
+  String get formatAlignRight => 'Align right';
+
+  @override
+  String get formatBold => 'Bold';
+
+  @override
+  String get formatItalic => 'Italic';
+
+  @override
+  String get formatUnderline => 'Underline';
+
+  @override
+  String get formatSuperscript => 'Superscript';
+
+  @override
+  String get formatSubscript => 'Subscript';
+
   @override
   String get vocabularyReading => 'Reading';