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/engine/stroke_predictor.dart

@@ -11,7 +11,7 @@ class PredictedPoint {
 }
 
 class StrokePredictor {
-  StrokePredictor({this.lookaheadMs = 12});
+  StrokePredictor({this.lookaheadMs = 8});
 
   /// How far ahead to project, in milliseconds of recent velocity.
   final double lookaheadMs;