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/input/pen_config.dart

@@ -18,6 +18,9 @@ enum PenButtonAction {
   undo,
   toggleTool,
   pan,
+
+  /// Hold to temporarily enable PDF text selection (OneNote-style).
+  selectText,
 }
 
 /// Immutable configuration for pen input behaviour.