feat(eraser): configurable size + stroke-eraser mode

"优化橡皮擦工具，你优化在哪" — the eraser already did segment erase, but
the radius was a hardcoded const with no size control and no whole-stroke
mode. Add both, OneNote/Notability-style:

- PenConfig: eraserRadius (0.005-0.1, default 0.02) + eraserWholeStroke
  bool, with copyWith / JSON / setters.
- PenCanvas: uses widget.eraserRadius for the live hit area AND the cursor
  preview (they stay in sync); eraserWholeStroke=true removes the whole
  stroke on contact, false keeps the segment-split behavior.
- pen_editor threads both from PenConfig.
- pen-settings: new Eraser section — size slider + "Stroke Eraser" switch.

Tests: pen_eraser_mode_widget proves point-eraser keeps the untouched ends
while stroke-eraser deletes the whole stroke from the same pass.

flutter analyze: 0 issues.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/canvas/pen_editor_screen.dart

@@ -564,6 +564,9 @@ class _PenEditorScreenState extends State<PenEditorScreen> {
             // natural default when no config is loaded yet.
             pressureGamma:
                 _penConfig?.value.pressureGamma ?? kNaturalPressureGamma,
+            eraserRadius:
+                _penConfig?.value.eraserRadius ?? kDefaultEraserRadius,
+            eraserWholeStroke: _penConfig?.value.eraserWholeStroke ?? false,
             sideButtonAction:
                 _penConfig?.value.sideButton ?? PenButtonAction.eraser,
             eraserEndAction: