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_slide_screen.dart

@@ -93,13 +93,7 @@ class _PenSlideScreenState extends State<PenSlideScreen> {
   static const double _highlighterWidthFraction = 0.02;
   static const Size _fallbackSlide = Size(1600, 900);
 
-  static const List<Color> _palette = [
-    Colors.black,
-    Colors.red,
-    Colors.blue,
-    Colors.green,
-    Colors.orange,
-  ];
+  static const List<Color> _palette = kInkPalette;
 
   int get _slideCount => widget.slideImagePaths.length;
   List<PenStroke> get _currentStrokes => _strokesBySlide[_slideIndex] ?? const [];