fix: Surface pen pressure, zoom glitches, sticky notes, selection UX

Wire Win32 pressure into Dart, tighten pinch guards, use geometric shape
strokes, expand the ink palette, and replace scratch-link split view with
an on-page sticky that shares the sidecar repo.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/editor/canvas/pen_note_screen.dart

@@ -111,13 +111,7 @@ class _PenNoteScreenState extends ConsumerState<PenNoteScreen> {
   static const double _penWidthFraction = 0.006;
   static const double _highlighterWidthFraction = 0.02;
 
-  static const List<Color> _palette = [
-    Colors.black,
-    Colors.red,
-    Colors.blue,
-    Colors.green,
-    Colors.orange,
-  ];
+  static const List<Color> _palette = kInkPalette;
 
   @override
   void initState() {