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>

lib/editor/canvas/pen_palette_widgets.dart

@@ -87,6 +87,64 @@ IconData shapeIcon(ShapeKind kind) => switch (kind) {
       ShapeKind.arrow => Icons.arrow_outward,
     };
 
+/// OneNote-style pen slot: each brush is its own toolbar button with a color
+/// underline (per-brush remembered color). Prefer this over [BrushPickerButton]
+/// when the UX wants pens visible side-by-side.
+class PenSlotButton extends StatelessWidget {
+  const PenSlotButton({
+    super.key,
+    required this.kind,
+    required this.selected,
+    required this.color,
+    required this.tooltip,
+    required this.onPressed,
+  });
+
+  final BrushKind kind;
+  final bool selected;
+  final Color color;
+  final String tooltip;
+  final VoidCallback onPressed;
+
+  @override
+  Widget build(BuildContext context) {
+    final cs = Theme.of(context).colorScheme;
+    final iconColor =
+        selected ? cs.onSecondaryContainer : cs.onSurfaceVariant;
+    return Tooltip(
+      message: tooltip,
+      child: InkWell(
+        onTap: onPressed,
+        borderRadius: BorderRadius.circular(20),
+        child: AnimatedContainer(
+          duration: const Duration(milliseconds: 150),
+          margin: const EdgeInsets.symmetric(horizontal: 2),
+          padding: const EdgeInsets.fromLTRB(6, 8, 6, 6),
+          decoration: BoxDecoration(
+            color: selected ? cs.secondaryContainer : Colors.transparent,
+            borderRadius: BorderRadius.circular(20),
+          ),
+          child: Column(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              Icon(brushIcon(kind), size: 22, color: iconColor),
+              const SizedBox(height: 3),
+              Container(
+                width: 16,
+                height: 3,
+                decoration: BoxDecoration(
+                  color: color,
+                  borderRadius: BorderRadius.circular(2),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}
+
 /// A dropdown that selects the active PEN brush (fountain / ballpoint / pencil).
 ///
 /// Highlighter and eraser remain separate tools. Tapping the button opens a