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/screens/split_view_screen.dart

@@ -89,13 +89,7 @@ class _SplitViewState extends State<SplitViewScreen> {
   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;
 
   // -- Auto-save debounce --
   Timer? _saveTimer;
@@ -119,12 +113,14 @@ class _SplitViewState extends State<SplitViewScreen> {
   @override
   void dispose() {
     _saveTimer?.cancel();
-    _saveImmediate();
-    final repo = _repo;
-    if (repo != null) {
-      repo.flush(); // fire-and-forget; atomic write finishes off the tree
-      repo.dispose();
+    // Schedule a final flush; retain-counted repo may still be held by the
+    // PDF editor, so dispose only drops our retain.
+    if (_dirty) {
+      unawaited(_saveImmediate());
+    } else {
+      unawaited(_repo?.flush() ?? Future<void>.value());
     }
+    _repo?.dispose();
     // PdfViewerController (pdfrx) has no dispose(); it detaches with the viewer.
     _scratchTransform.dispose();
     super.dispose();
@@ -166,9 +162,9 @@ class _SplitViewState extends State<SplitViewScreen> {
 
   Future<void> _saveImmediate() async {
     if (!_dirty) return;
-    _dirty = false;
     final repo = _repo;
     if (repo == null) return;
+    // Keep dirty until schedule succeeds so a race during load can't swallow ink.
     repo.scheduleScratchpadSave(
       widget.scratchLinkId,
       SidecarScratchpad(
@@ -177,6 +173,7 @@ class _SplitViewState extends State<SplitViewScreen> {
         strokes: List<InkStroke>.of(_strokes),
       ),
     );
+    _dirty = false;
     await repo.flush();
   }
 
@@ -295,9 +292,9 @@ class _SplitViewState extends State<SplitViewScreen> {
         title: const Text('Scratch link', style: TextStyle(fontSize: 16)),
         leading: IconButton(
           icon: const Icon(Icons.arrow_back),
-          onPressed: () {
-            _saveImmediate();
-            Navigator.of(context).pop();
+          onPressed: () async {
+            await _saveImmediate();
+            if (context.mounted) Navigator.of(context).pop();
           },
         ),
         actions: [