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/home_screen.dart

@@ -7,10 +7,9 @@ import '../providers/document_provider.dart';
 import '../providers/note_provider.dart';
 import '../providers/ocr_provider.dart';
 import '../editor/canvas/pen_editor_screen.dart';
-import '../editor/pdf/spike_launcher.dart';
 import '../services/pdf_service.dart';
 import '../services/pptx_service.dart';
-import 'note_editor_screen.dart';
+import '../editor/canvas/pen_note_screen.dart';
 import 'ppt_annotator_screen.dart';
 import 'search_screen.dart';
 import 'settings_screen.dart';
@@ -70,12 +69,6 @@ class HomeScreen extends ConsumerWidget {
               ).push(MaterialPageRoute(builder: (_) => const SearchScreen()));
             },
           ),
-          // New pen-first canvas editor (beta).
-          IconButton(
-            icon: const Icon(Icons.draw_outlined),
-            tooltip: l.penCanvasBeta,
-            onPressed: () => openM1Spike(context),
-          ),
         ],
       ),
       floatingActionButton: FloatingActionButton(
@@ -196,7 +189,7 @@ class HomeScreen extends ConsumerWidget {
     if (context.mounted) {
       Navigator.of(
         context,
-      ).push(MaterialPageRoute(builder: (_) => NoteEditorScreen(note: note)));
+      ).push(MaterialPageRoute(builder: (_) => PenNoteScreen(note: note)));
     }
   }
 
@@ -375,7 +368,7 @@ class _NoteTileState extends ConsumerState<_NoteTile> {
           ),
           onTap: () {
             Navigator.of(context).push(
-              MaterialPageRoute(builder: (_) => NoteEditorScreen(note: note)),
+              MaterialPageRoute(builder: (_) => PenNoteScreen(note: note)),
             );
           },
           onLongPress: () => _confirmDelete(context),
@@ -425,7 +418,7 @@ class _NoteTileState extends ConsumerState<_NoteTile> {
     if (!mounted) return;
     if (result == 'open') {
       Navigator.of(this.context).push(
-        MaterialPageRoute(builder: (_) => NoteEditorScreen(note: widget.note)),
+        MaterialPageRoute(builder: (_) => PenNoteScreen(note: widget.note)),
       );
     } else if (result == 'delete') {
       _confirmDelete(this.context);