fix: PDF finger ink, chrome UX, OneNote pens, and pen physics

Wire finger drawing on PDF without breaking pinch; auto-hide page scrubber and fix bounce; share sticky tools with resize and per-page remember; side-button select; separate pen slots with colors; rnote pressure shapes plus tip-velocity width and lower stroke latency.

Co-authored-by: Cursor <cursoragent@cursor.com>

test/pen_physics_test.dart

@@ -0,0 +1,34 @@
+// Tests for tipVelocityWidthScale (physical tip model).
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/engine/brush.dart';
+import 'package:badnote/editor/engine/pen_physics.dart';
+
+void main() {
+  group('tipVelocityWidthScale', () {
+    test('idle speed leaves all brushes at 1.0', () {
+      for (final k in BrushKind.values) {
+        expect(tipVelocityWidthScale(k, 0.0), closeTo(1.0, 1e-9));
+      }
+    });
+
+    test('fountain thins more than ballpoint at the same speed', () {
+      const speed = 2.0; // reference fast handwriting
+      final fountain = tipVelocityWidthScale(BrushKind.fountainPen, speed);
+      final ballpoint = tipVelocityWidthScale(BrushKind.ballpoint, speed);
+      expect(fountain, closeTo(0.85, 1e-9));
+      expect(ballpoint, closeTo(0.98, 1e-9));
+      expect(fountain, lessThan(ballpoint));
+    });
+
+    test('highlighter ignores velocity', () {
+      expect(tipVelocityWidthScale(BrushKind.highlighter, 5.0), 1.0);
+    });
+
+    test('NaN / negative speed treated as idle', () {
+      expect(tipVelocityWidthScale(BrushKind.fountainPen, double.nan), 1.0);
+      expect(tipVelocityWidthScale(BrushKind.fountainPen, -1.0), 1.0);
+    });
+  });
+}