fix(canvas): persist strokes, pressure, slider

Strokes vanished on pen-up: StaticInkPainter aliased the same mutable
list so shouldRepaint saw no change. Commit/erase now replace the list.
Finger-drawing toggle wins over palm-rejection; pressure surfaces even
when the pen reports no min/max range; pages recenter after a flip; the
keyboard page-jump (unreliable on Windows) is now a drag slider. Also
register the dynamic_color plugin in generated registrants.

lib/editor/canvas/pen_canvas.dart

@@ -93,16 +93,15 @@ class _PenCanvasState extends State<PenCanvas> {
   /// Live stroke snapshot handed to the LiveInkPainter; null when idle.
   PenStroke? _liveStroke;
 
-  /// True once any stylus event is seen this session → finger-drawing forced
-  /// off so a resting palm pans instead of marking.
-  bool _stylusSeen = false;
-
   /// True when the active stylus reports the eraser signal (barrel button or
   /// inverted stylus), detected on hover/down.
   bool _eraserActive = false;
 
-  bool get _fingerDrawingEnabled =>
-      widget.allowFingerDrawing && !_stylusSeen;
+  // The explicit user toggle wins: if finger-drawing is ON, a single finger
+  // draws even after a stylus has been seen. (Palm rejection when the toggle is
+  // OFF is automatic — fingers simply never draw — and a 2nd pointer always
+  // cancels an in-progress stroke regardless.)
+  bool get _fingerDrawingEnabled => widget.allowFingerDrawing;
 
   bool _isStylus(PointerDeviceKind kind) =>
       kind == PointerDeviceKind.stylus ||
@@ -112,9 +111,17 @@ class _PenCanvasState extends State<PenCanvas> {
   /// usable pressure range (then perfect_freehand simulates pressure).
   double? _normalizedPressure(PointerEvent event) {
     if (!_isStylus(event.kind)) return null;
-    if (event.pressureMin == event.pressureMax) return null;
     final range = event.pressureMax - event.pressureMin;
-    return ((event.pressure - event.pressureMin) / range).clamp(0.0, 1.0);
+    if (range > 0.0001) {
+      return ((event.pressure - event.pressureMin) / range).clamp(0.0, 1.0);
+    }
+    // No advertised range (some Windows pen stacks): use the raw normalized
+    // pressure directly if it's a usable non-degenerate value, so we still get
+    // real force instead of falling back to velocity simulation.
+    if (event.pressure > 0.0 && event.pressure < 1.0) {
+      return event.pressure;
+    }
+    return null;
   }
 
   /// The eraser signal: barrel/secondary button held, or an inverted stylus.
@@ -244,7 +251,6 @@ class _PenCanvasState extends State<PenCanvas> {
 
   void _onPointerHover(PointerHoverEvent event) {
     if (_isStylus(event.kind)) {
-      _stylusSeen = true;
       // Detect eraser (barrel button / inverted) while hovering.
       _eraserActive = _isEraserSignal(event);
     }
@@ -252,7 +258,6 @@ class _PenCanvasState extends State<PenCanvas> {
 
   void _onPointerDown(PointerDownEvent event) {
     if (event.kind == PointerDeviceKind.trackpad) return;
-    if (_isStylus(event.kind)) _stylusSeen = true;
 
     _activePointers[event.pointer] = event.kind;