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/input/pen_config.dart

@@ -7,6 +7,10 @@ import 'package:shared_preferences/shared_preferences.dart';
 import '../engine/stroke_geometry.dart' show kDefaultPenThinning;
 import 'pressure_curve.dart' show kNaturalPressureGamma;
 
+/// Default eraser radius as a fraction of page width (the legacy fixed value,
+/// now the default of the configurable [PenConfig.eraserRadius]).
+const double kDefaultEraserRadius = 0.02;
+
 /// Action that can be triggered by a hardware pen button or the eraser end.
 enum PenButtonAction {
   none,
@@ -29,12 +33,16 @@ class PenConfig {
     this.penWidth = 0.004,
     this.highlighterWidth = 0.02,
     this.pressureSensitivity = kDefaultPenThinning,
+    this.eraserRadius = kDefaultEraserRadius,
+    this.eraserWholeStroke = false,
   })  : assert(pressureGamma >= 0.3 && pressureGamma <= 3.0,
             'pressureGamma must be in [0.3, 3.0]'),
         assert(palmRejectionMs >= 0.0 && palmRejectionMs <= 500.0,
             'palmRejectionMs must be in [0, 500]'),
         assert(pressureSensitivity >= 0.0 && pressureSensitivity <= 1.0,
-            'pressureSensitivity must be in [0, 1]');
+            'pressureSensitivity must be in [0, 1]'),
+        assert(eraserRadius >= 0.005 && eraserRadius <= 0.1,
+            'eraserRadius must be in [0.005, 0.1]');
 
   /// Which action fires when the side barrel button is held.
   final PenButtonAction sideButton;
@@ -67,6 +75,16 @@ class PenConfig {
   /// export golden are unchanged.
   final double pressureSensitivity;
 
+  /// Eraser radius as a fraction of page width. Range [0.005, 0.1], default
+  /// [kDefaultEraserRadius]. Controls both the live erase hit area and the
+  /// on-screen eraser cursor.
+  final double eraserRadius;
+
+  /// When true the eraser removes a WHOLE stroke on contact (OneNote-style
+  /// stroke eraser); when false it does a partial / segment erase (the default,
+  /// rnote-style point eraser).
+  final bool eraserWholeStroke;
+
   PenConfig copyWith({
     PenButtonAction? sideButton,
     PenButtonAction? eraserEnd,
@@ -76,6 +94,8 @@ class PenConfig {
     double? penWidth,
     double? highlighterWidth,
     double? pressureSensitivity,
+    double? eraserRadius,
+    bool? eraserWholeStroke,
   }) {
     return PenConfig(
       sideButton: sideButton ?? this.sideButton,
@@ -86,6 +106,8 @@ class PenConfig {
       penWidth: penWidth ?? this.penWidth,
       highlighterWidth: highlighterWidth ?? this.highlighterWidth,
       pressureSensitivity: pressureSensitivity ?? this.pressureSensitivity,
+      eraserRadius: eraserRadius ?? this.eraserRadius,
+      eraserWholeStroke: eraserWholeStroke ?? this.eraserWholeStroke,
     );
   }
 
@@ -98,6 +120,8 @@ class PenConfig {
         'penWidth': penWidth,
         'highlighterWidth': highlighterWidth,
         'pressureSensitivity': pressureSensitivity,
+        'eraserRadius': eraserRadius,
+        'eraserWholeStroke': eraserWholeStroke,
       };
 
   factory PenConfig.fromJson(Map<String, dynamic> json) {
@@ -116,6 +140,9 @@ class PenConfig {
       highlighterWidth: (json['highlighterWidth'] as num?)?.toDouble() ?? 0.02,
       pressureSensitivity:
           (json['pressureSensitivity'] as num?)?.toDouble() ?? kDefaultPenThinning,
+      eraserRadius:
+          (json['eraserRadius'] as num?)?.toDouble() ?? kDefaultEraserRadius,
+      eraserWholeStroke: json['eraserWholeStroke'] as bool? ?? false,
     );
   }
 
@@ -131,7 +158,9 @@ class PenConfig {
           fingerDrawing == other.fingerDrawing &&
           penWidth == other.penWidth &&
           highlighterWidth == other.highlighterWidth &&
-          pressureSensitivity == other.pressureSensitivity;
+          pressureSensitivity == other.pressureSensitivity &&
+          eraserRadius == other.eraserRadius &&
+          eraserWholeStroke == other.eraserWholeStroke;
 
   @override
   int get hashCode => Object.hash(
@@ -143,6 +172,8 @@ class PenConfig {
         penWidth,
         highlighterWidth,
         pressureSensitivity,
+        eraserRadius,
+        eraserWholeStroke,
       );
 }
 
@@ -253,6 +284,18 @@ class PenConfigController extends ChangeNotifier {
   }
 
   /// Sets [PenConfig.pressureSensitivity]. Clamped to [0, 1].
+  /// Sets [PenConfig.eraserRadius]. Clamped to [0.005, 0.1].
+  Future<void> setEraserRadius(double radius) async {
+    _value = _value.copyWith(eraserRadius: radius.clamp(0.005, 0.1));
+    await _persist();
+  }
+
+  /// Sets [PenConfig.eraserWholeStroke] (true = OneNote-style stroke eraser).
+  Future<void> setEraserWholeStroke(bool whole) async {
+    _value = _value.copyWith(eraserWholeStroke: whole);
+    await _persist();
+  }
+
   Future<void> setPressureSensitivity(double sensitivity) async {
     _value = _value.copyWith(pressureSensitivity: sensitivity.clamp(0.0, 1.0));
     notifyListeners();