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,
   ),