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/ui/pen_settings_page.dart

@@ -161,6 +161,31 @@ class _PenSettingsSheet extends StatelessWidget {
                   formatValue: (v) => v.toStringAsFixed(4),
                   onChanged: controller.setHighlighterWidth,
                 ),
+
+                // ── Eraser ────────────────────────────────────────────────
+                _SectionHeader(
+                  title: 'Eraser',
+                  icon: Icons.cleaning_services_outlined,
+                  colorScheme: colorScheme,
+                ),
+                _SliderTile(
+                  label: 'Eraser Size',
+                  value: config.eraserRadius,
+                  min: 0.005,
+                  max: 0.1,
+                  divisions: 19,
+                  formatValue: (v) => v.toStringAsFixed(3),
+                  onChanged: controller.setEraserRadius,
+                ),
+                SwitchListTile(
+                  title: const Text('Stroke Eraser'),
+                  subtitle: const Text(
+                    'Erase a whole stroke on contact (off: erase by segment)',
+                  ),
+                  value: config.eraserWholeStroke,
+                  onChanged: controller.setEraserWholeStroke,
+                  contentPadding: EdgeInsets.zero,
+                ),
               ],
             );
           },