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_palette_widgets.dart

@@ -10,6 +10,22 @@ import '../../l10n/app_localizations.dart';
 import '../engine/brush.dart';
 import 'editor_tool.dart';
 
+/// Shared ink color palette for PDF / note / slide / scratch editors.
+const List<Color> kInkPalette = <Color>[
+  Color(0xFF1A1A1A),
+  Color(0xFFC62828),
+  Color(0xFF1565C0),
+  Color(0xFF2E7D32),
+  Color(0xFFEF6C00),
+  Color(0xFF6A1B9A),
+  Color(0xFF00838F),
+  Color(0xFF5D4037),
+  Color(0xFFF9A825),
+  Color(0xFFE91E63),
+  Color(0xFF455A64),
+  Color(0xFF37474F),
+];
+
 /// Localized display name for a brush (single source so all three editors agree).
 String brushLabel(BrushKind kind, AppLocalizations l) => switch (kind) {
       BrushKind.fountainPen => l.brushFountainPen,