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/editor/notebook/ink_stroke_adapter.dart

@@ -0,0 +1,96 @@
+// lib/editor/notebook/ink_stroke_adapter.dart
+//
+// Bridge between the legacy note/ppt storage model (`InkStroke`, ABSOLUTE pixel
+// coordinates, `PenTool`) and the pen-first canvas model (`PenStroke`,
+// NORMALIZED [0,1] coordinates, `PenStrokeKind`). The pen-first canvas is the
+// single performant inking engine, so notes and slides are rebuilt on top of it
+// and persisted back as `InkStroke` via this adapter.
+//
+// Coordinates are normalized against a logical page rectangle: ink absolute
+// (x,y) -> pen (x/pageW, y/pageH) and back. Stroke width is likewise expressed
+// as a fraction of the page width on the pen side and as absolute pixels on the
+// ink side. Only freehand pen/highlighter strokes round-trip; shape/text
+// `PenTool`s have no pen-canvas representation and are dropped (the pen-first
+// note is handwriting-first — see the rebuild roadmap).
+
+import 'dart:ui' show Size;
+
+import '../../models/ink_point.dart';
+import '../../models/ink_stroke.dart';
+import '../../models/pen_tool.dart';
+import '../canvas/pen_stroke.dart';
+
+/// Logical page rectangle a blank note is inked on (portrait, ~A4 √2 ratio).
+/// Strokes are normalized against this so they stay pinned under zoom/pan.
+const Size kNoteLogicalPage = Size(1000, 1414);
+
+/// True when [tool] is a freehand mark the pen canvas can render
+/// (pen/marker/highlighter). Shapes and text are not representable.
+bool isFreehandTool(PenTool tool) =>
+    tool == PenTool.pen ||
+    tool == PenTool.marker ||
+    tool == PenTool.highlighter;
+
+/// Maps an ink [PenTool] to the pen-canvas stroke kind.
+PenStrokeKind penKindFromTool(PenTool tool) =>
+    tool == PenTool.highlighter ? PenStrokeKind.highlighter : PenStrokeKind.pen;
+
+/// Maps a pen-canvas stroke kind back to a [PenTool].
+PenTool toolFromPenKind(PenStrokeKind kind) =>
+    kind == PenStrokeKind.highlighter ? PenTool.highlighter : PenTool.pen;
+
+/// Convert a stored [InkStroke] (absolute px on [page]) to a [PenStroke]
+/// (normalized). Returns null for non-freehand strokes (shapes/text), which the
+/// pen canvas cannot draw.
+PenStroke? penStrokeFromInk(InkStroke s, Size page) {
+  if (!isFreehandTool(s.tool)) return null;
+  if (s.points.isEmpty) return null;
+  final w = page.width <= 0 ? 1.0 : page.width;
+  final h = page.height <= 0 ? 1.0 : page.height;
+  return PenStroke(
+    points: [
+      for (final p in s.points)
+        PenPoint(p.x / w, p.y / h, p.pressure, tilt: p.tilt),
+    ],
+    color: s.color,
+    width: s.strokeWidth / w,
+    kind: penKindFromTool(s.tool),
+  );
+}
+
+/// Convert a freshly drawn [PenStroke] (normalized) back to an [InkStroke]
+/// (absolute px on [page]) for persistence. [id] and [createdAt] come from the
+/// caller (uuid + clock) so this stays pure/deterministic.
+InkStroke inkStrokeFromPen(
+  PenStroke s,
+  Size page, {
+  required String id,
+  required DateTime createdAt,
+}) {
+  final w = page.width <= 0 ? 1.0 : page.width;
+  final h = page.height <= 0 ? 1.0 : page.height;
+  return InkStroke(
+    id: id,
+    points: [
+      for (final p in s.points)
+        InkPoint(
+          x: p.x * w,
+          y: p.y * h,
+          pressure: p.pressure ?? 0.5,
+          tilt: p.tilt ?? 0.0,
+          timestamp: 0,
+        ),
+    ],
+    tool: toolFromPenKind(s.kind),
+    color: s.color,
+    strokeWidth: s.width * w,
+    createdAt: createdAt,
+  );
+}
+
+/// Convert a list of stored ink strokes to pen strokes, dropping the ones the
+/// canvas cannot represent (shapes/text). Order is preserved.
+List<PenStroke> penStrokesFromInk(Iterable<InkStroke> strokes, Size page) =>
+    [for (final s in strokes) penStrokeFromInk(s, page)]
+        .whereType<PenStroke>()
+        .toList();