feat(note): rebuild note editor on the pen-first canvas

Notes now use the single performant inking engine (PenCanvas) instead of
the old ink_canvas, per "all note features on the pen-first canvas".

- ink_stroke_adapter: pure InkStroke<->PenStroke bridge (normalize against
  a logical note page; drop non-freehand shapes/text). Round-trip tested.
- pen_palette_widgets: shared M3 ToolButton/PaletteDivider/RoundIconButton
  so PDF + note editors use identical chrome (PenEditorScreen migrated to
  them; its private copies deleted).
- PenNoteScreen: PenCanvas over a white logical page, undo/redo, title,
  save -> Note.strokes (+ local OCR for search). Pressure curve, eraser
  size/mode and palm rejection all inherited from the shared canvas.
- Route home (new/open) + search note hits -> PenNoteScreen; remove the
  now-redundant "Pen Canvas (beta)" spike button; delete the dead old
  note_editor_screen.

Tests: ink_stroke_adapter (5) + pen_note_screen widget (load + commit, 2).
flutter analyze: 0 issues. Full suite: 265/265.

lib/screens/search_screen.dart

@@ -7,7 +7,7 @@ import '../editor/canvas/pen_editor_screen.dart';
 import '../l10n/app_localizations.dart';
 import '../models/note.dart';
 import '../providers/search_provider.dart';
-import 'note_editor_screen.dart';
+import '../editor/canvas/pen_note_screen.dart';
 
 class SearchScreen extends ConsumerStatefulWidget {
   const SearchScreen({super.key});
@@ -179,7 +179,7 @@ class _NoteSearchResultTile extends StatelessWidget {
       onTap: () {
         Navigator.of(
           context,
-        ).push(MaterialPageRoute(builder: (_) => NoteEditorScreen(note: note)));
+        ).push(MaterialPageRoute(builder: (_) => PenNoteScreen(note: note)));
       },
     );
   }