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/ui/pen_settings_page.dart

@@ -270,6 +270,7 @@ class _ActionDropdown extends StatelessWidget {
         PenButtonAction.undo => 'Undo',
         PenButtonAction.toggleTool => 'Toggle Tool',
         PenButtonAction.pan => 'Pan',
+        PenButtonAction.selectText => 'Select text',
       };
 
   @override