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_interactive_viewer.dart

@@ -46,13 +46,14 @@ const Set<PointerDeviceKind> _kPanZoomDevices = <PointerDeviceKind>{
 /// A real pinch changes scale only modestly per frame (≲1.15x at 60fps). A frame
 /// demanding far more than this is a Windows multi-touch position glitch, not
 /// intent — that frame is dropped so the zoom can't pop and snap back.
-const double _kScaleGlitchHi = 1.4;
+/// Device logs showed spikes ~1.30; keep the band under that so jumps die.
+const double _kScaleGlitchHi = 1.18;
 const double _kScaleGlitchLo = 1 / _kScaleGlitchHi;
 
 /// During a 2-finger gesture the focal point (finger midpoint) should move
 /// smoothly. A single-frame local jump beyond this is a Windows touch misread,
 /// and the frame is dropped (position-jump guard).
-const double _kFocalGlitchPx = 250.0;
+const double _kFocalGlitchPx = 100.0;
 
 const double _kDrag = 0.0000135;