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/canvas/pen_palette_widgets.dart

@@ -0,0 +1,96 @@
+// lib/editor/canvas/pen_palette_widgets.dart
+//
+// Shared Material 3 chrome for the pen-first editors (PDF, note, slide) so the
+// floating tool palette looks and behaves identically everywhere — one source
+// of truth for the inking UI.
+
+import 'package:flutter/material.dart';
+
+/// A Material 3 toggle-style icon button for the floating tool palette.
+class ToolButton extends StatelessWidget {
+  const ToolButton({
+    super.key,
+    required this.icon,
+    required this.selected,
+    required this.tooltip,
+    required this.onPressed,
+  });
+
+  final IconData icon;
+  final bool selected;
+  final String tooltip;
+
+  /// Tap handler. When null the button renders disabled (dimmed, no ripple).
+  final VoidCallback? onPressed;
+
+  @override
+  Widget build(BuildContext context) {
+    final cs = Theme.of(context).colorScheme;
+    final enabled = onPressed != null;
+    final iconColor = !enabled
+        ? cs.onSurfaceVariant.withValues(alpha: 0.38)
+        : selected
+            ? cs.onSecondaryContainer
+            : cs.onSurfaceVariant;
+    return Tooltip(
+      message: tooltip,
+      child: InkWell(
+        borderRadius: BorderRadius.circular(20),
+        onTap: onPressed,
+        child: AnimatedContainer(
+          duration: const Duration(milliseconds: 150),
+          margin: const EdgeInsets.symmetric(horizontal: 2),
+          padding: const EdgeInsets.all(8),
+          decoration: BoxDecoration(
+            color: selected ? cs.secondaryContainer : Colors.transparent,
+            borderRadius: BorderRadius.circular(20),
+          ),
+          child: Icon(icon, size: 22, color: iconColor),
+        ),
+      ),
+    );
+  }
+}
+
+/// A thin vertical divider between palette groups.
+class PaletteDivider extends StatelessWidget {
+  const PaletteDivider({super.key, required this.cs});
+  final ColorScheme cs;
+  @override
+  Widget build(BuildContext context) => Container(
+        width: 1,
+        height: 24,
+        margin: const EdgeInsets.symmetric(horizontal: 6),
+        color: cs.outlineVariant,
+      );
+}
+
+/// A round, tonal icon button (used for the floating back button).
+class RoundIconButton extends StatelessWidget {
+  const RoundIconButton({
+    super.key,
+    required this.icon,
+    required this.tooltip,
+    required this.onPressed,
+  });
+
+  final IconData icon;
+  final String tooltip;
+  final VoidCallback onPressed;
+
+  @override
+  Widget build(BuildContext context) {
+    final cs = Theme.of(context).colorScheme;
+    return Material(
+      color: cs.surfaceContainerHigh,
+      elevation: 3,
+      shape: const CircleBorder(),
+      child: IconButton(
+        tooltip: tooltip,
+        icon: Icon(icon),
+        color: cs.onSurfaceVariant,
+        onPressed: onPressed,
+      ),
+    );
+  }
+}