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/brush.dart

@@ -142,13 +142,15 @@ const Map<BrushKind, BrushProfile> kBrushPresets = {
     kind: BrushKind.fountainPen,
     baseWidthFraction: 0.006,
     pressureGamma: 2.0,
-    pfThinning: 0.9,
-    pfStreamline: 0.45,
-    pfSmoothing: 0.55,
+    // Was 0.9 — too aggressive on short CJK strokes (width collapses mid-glyph).
+    pfThinning: 0.65,
+    pfStreamline: 0.4,
+    pfSmoothing: 0.5,
     simulatePressure: false,
     capStart: true,
     capEnd: true,
-    taper: true,
+    // Light taper only; full taper made Chinese characters look frayed.
+    taper: false,
     opacity: 1.0,
     blendMultiply: false,
   ),

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.