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_canvas.dart

@@ -58,6 +58,8 @@ class PenCanvas extends StatefulWidget {
     this.thinning = kDefaultPenThinning,
     this.pressureGamma = kNaturalPressureGamma,
     this.pressureFloor = kNaturalPressureFloor,
+    this.eraserRadius = kDefaultEraserRadius,
+    this.eraserWholeStroke = false,
     this.sideButtonAction = PenButtonAction.eraser,
     this.eraserEndAction = PenButtonAction.eraser,
     this.onPenButtonAction,
@@ -115,6 +117,15 @@ class PenCanvas extends StatefulWidget {
   /// scratchy near-zero width. From `PenConfig.pressureFloor`.
   final double pressureFloor;
 
+  /// Eraser radius as a fraction of page width (live hit area + cursor size).
+  /// From `PenConfig.eraserRadius`.
+  final double eraserRadius;
+
+  /// When true the eraser removes a whole stroke on contact (OneNote-style);
+  /// when false it does a partial / segment erase. From
+  /// `PenConfig.eraserWholeStroke`.
+  final bool eraserWholeStroke;
+
   /// Configured action for the pen's side barrel button (W3 — resolved against
   /// the native pen plugin's flags on Windows).
   final PenButtonAction sideButtonAction;
@@ -180,12 +191,6 @@ class _PenCanvasState extends State<PenCanvas> {
   bool get _isEraserMode =>
       widget.tool == CanvasTool.eraser || _eraserActive;
 
-  /// Eraser radius as a fraction of page width (shared by the live erase and the
-  /// preview overlay so they always agree). A decisive fixed size — the old
-  /// strokeWidth*2 was so small that a pass removed only a couple of points and
-  /// the stroke visibly survived ("选中了的笔画也不见得能删掉").
-  static const double _eraserRadius = 0.02;
-
   /// Page aspect (height / width) so the eraser circle stays round on screen.
   double get _pageAspect => widget.pageSize.width <= 0
       ? 1.0
@@ -416,13 +421,16 @@ class _PenCanvasState extends State<PenCanvas> {
   /// stays round on screen (the page rect is not square).
   void _eraseAt(PenPoint? p) {
     if (p == null) return;
-    final radius = _eraserRadius; // normalized (page-width fraction)
+    final radius = widget.eraserRadius; // normalized (page-width fraction)
     final aspect = _pageAspect;
     for (var i = widget.strokes.length - 1; i >= 0; i--) {
       final stroke = widget.strokes[i];
       if (!strokeHit(stroke, p.x, p.y, radius, aspect: aspect)) continue;
-      final pieces =
-          splitStrokeByCircle(stroke, p.x, p.y, radius, aspect: aspect);
+      // Stroke-eraser mode: a hit removes the entire stroke (empty replacement).
+      // Point-eraser mode (default): cut out the touched span, keep the rest.
+      final pieces = widget.eraserWholeStroke
+          ? const <PenStroke>[]
+          : splitStrokeByCircle(stroke, p.x, p.y, radius, aspect: aspect);
       // Defensive no-op guard (strokeHit already passed, so a hit is expected).
       if (pieces.length == 1 && identical(pieces.first, stroke)) return;
       widget.onEraseStroke(i, pieces);
@@ -584,7 +592,7 @@ class _PenCanvasState extends State<PenCanvas> {
                       painter: EraserPreviewPainter(
                         strokes: widget.strokes,
                         cursor: _eraserCursor,
-                        radius: _eraserRadius,
+                        radius: widget.eraserRadius,
                         aspect: _pageAspect,
                         pageSize: widget.pageSize,
                       ),

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: