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/engine/shape_geometry.dart

@@ -13,6 +13,7 @@ import 'dart:math' as math;
 
 import '../canvas/editor_tool.dart';
 import '../canvas/pen_stroke.dart';
+import 'brush.dart';
 
 /// Number of points sampled around an ellipse. Kept as a const so tests can pin
 /// it (spec: "ellipse = sampled points ~48"). The polyline is closed, so the
@@ -23,6 +24,10 @@ const int kEllipseSamples = 48;
 /// width (no pressure taper for geometric shapes).
 const double _kShapePressure = 1.0;
 
+/// Geometric shapes must NOT inherit fountain thinning/taper — force a near-
+/// constant-width brush so line/rect/ellipse look like ruler ink.
+const BrushKind kShapeBrush = BrushKind.ballpoint;
+
 /// Generate the normalized polyline for [kind] spanning [start] → [end].
 ///
 ///   * [ShapeKind.line]      → 2 points.