feat(learn): the assistant's button folded on Home too, the conversation at full height folding into it, and smaller fixes from the developer's list (ADR 0021, D64, D65)

- Home's assistant starts folded into its button at the lower left, as
  in the Library, where a field stood at its foot.
- The button has Material 3's floating action button shape (a rounded
  square) rather than a circle, its gradient ring kept.
- On a studyset below expanded windows it stands in one row with Add
  material and Study rather than above them.
- While studying, the ratings below the card make way while its field
  is open rather than standing under it.
- The conversation opens at the window's full height, its header above
  the list, so a drag on the header moves the sheet and a drag on the
  list scrolls it. Close ends the conversation; any other way out folds
  it into the button (ConversationDockCubit, above the router), its
  answer still coming, until any button opens it again or another
  conversation starts.
- An answer's Copy icon lines up with the answer's text.
- A sense's grammar notes run on after its glosses as one flowing text,
  so a short note such as "n" no longer takes a line of its own.
- A card's hints show their count in a grey badge beside the title, so
  it no longer reads as the hint's number.

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

apps/learn/halkyon_learn_flutter/test/feature/jellyfin/watch_screen_test.dart

@@ -719,7 +719,16 @@ void main() {
     await tester.pump(const Duration(milliseconds: 700));
     await tester.pumpAndSettle();
     expect(find.byType(VocabularySensesView), findsOneWidget);
-    expect(find.text('station, as in sense 6 of a long entry'), findsOneWidget);
+    expect(
+      find.descendant(
+        of: find.byType(VocabularySensesView),
+        matching: find.textContaining(
+          'station, as in sense 6 of a long entry',
+          findRichText: true,
+        ),
+      ),
+      findsOneWidget,
+    );
 
     // Left, it folds again.
     await mouse.moveTo(Offset.zero);