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.

test/pen_note_screen_widget_test.dart

@@ -0,0 +1,76 @@
+// Screen-level guard for the pen-first note editor: an existing note's ink
+// strokes load onto the PenCanvas via the adapter, and a stylus pass commits a
+// new stroke. Exercises the real PenNoteScreen (no DB needed — persistence only
+// runs on save).
+
+import 'package:flutter/gestures.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:badnote/editor/canvas/pen_canvas.dart';
+import 'package:badnote/editor/canvas/pen_note_screen.dart';
+import 'package:badnote/models/ink_point.dart';
+import 'package:badnote/models/ink_stroke.dart';
+import 'package:badnote/models/note.dart';
+import 'package:badnote/models/pen_tool.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  Note noteWith(List<InkStroke> strokes) => Note(
+        id: 'n1',
+        title: 'Test',
+        strokes: strokes,
+        createdAt: DateTime.fromMillisecondsSinceEpoch(0),
+        updatedAt: DateTime.fromMillisecondsSinceEpoch(0),
+      );
+
+  InkStroke freehand() => InkStroke(
+        id: 's1',
+        points: const [
+          InkPoint(x: 100, y: 200, timestamp: 0),
+          InkPoint(x: 300, y: 400, timestamp: 0),
+          InkPoint(x: 500, y: 700, timestamp: 0),
+        ],
+        tool: PenTool.pen,
+        createdAt: DateTime.fromMillisecondsSinceEpoch(0),
+      );
+
+  int canvasStrokeCount(WidgetTester tester) =>
+      tester.widget<PenCanvas>(find.byType(PenCanvas)).strokes.length;
+
+  testWidgets('loads an existing note\'s ink strokes onto the canvas',
+      (tester) async {
+    SharedPreferences.setMockInitialValues({});
+    await tester.pumpWidget(ProviderScope(
+      child: MaterialApp(home: PenNoteScreen(note: noteWith([freehand()]))),
+    ));
+    await tester.pump(); // let PenConfig load
+
+    expect(find.byType(PenCanvas), findsOneWidget);
+    expect(canvasStrokeCount(tester), 1,
+        reason: 'the note\'s freehand stroke should load via the adapter');
+  });
+
+  testWidgets('a stylus pass commits a new stroke', (tester) async {
+    SharedPreferences.setMockInitialValues({});
+    await tester.pumpWidget(ProviderScope(
+      child: MaterialApp(home: PenNoteScreen(note: noteWith([]))),
+    ));
+    await tester.pump();
+
+    expect(canvasStrokeCount(tester), 0);
+
+    final center = tester.getCenter(find.byType(PenCanvas));
+    final g = await tester.startGesture(center, kind: PointerDeviceKind.stylus);
+    await g.moveBy(const Offset(40, 30));
+    await g.moveBy(const Offset(30, 20));
+    await g.up();
+    await tester.pump();
+
+    expect(canvasStrokeCount(tester), 1,
+        reason: 'the drawn stroke should be committed to the canvas');
+  });
+}