Add eraser

lib/features/sheet_viewer/drawing/drawing_toolbar.dart

@@ -43,13 +43,24 @@ class DrawingToolbar extends StatelessWidget {
               ...PaintPreset.quickAccess.map((preset) => _buildPresetButton(
                     context,
                     preset,
-                    isSelected: controller.currentPreset == preset,
+                    isSelected: !controller.isEraserMode &&
+                        controller.currentPreset == preset,
                   )),
 
               const SizedBox(width: 8),
               _buildDivider(context),
               const SizedBox(width: 8),
 
+              // Eraser button
+              _buildEraserButton(
+                context,
+                isSelected: controller.isEraserMode,
+              ),
+
+              const SizedBox(width: 8),
+              _buildDivider(context),
+              const SizedBox(width: 8),
+
               // Undo button
               _buildActionButton(
                 context,
@@ -127,6 +138,37 @@ class DrawingToolbar extends StatelessWidget {
     );
   }
 
+  Widget _buildEraserButton(
+    BuildContext context, {
+    required bool isSelected,
+  }) {
+    final colorScheme = Theme.of(context).colorScheme;
+
+    return Tooltip(
+      message: 'Eraser',
+      child: InkWell(
+        onTap: () => controller.setEraserMode(true),
+        borderRadius: BorderRadius.circular(20),
+        child: Container(
+          width: 36,
+          height: 36,
+          margin: const EdgeInsets.symmetric(horizontal: 2),
+          decoration: BoxDecoration(
+            shape: BoxShape.circle,
+            border: isSelected
+                ? Border.all(color: colorScheme.primary, width: 2)
+                : null,
+          ),
+          child: Icon(
+            Icons.auto_fix_high,
+            size: 20,
+            color: colorScheme.onSurface,
+          ),
+        ),
+      ),
+    );
+  }
+
   Widget _buildActionButton(
     BuildContext context, {
     required IconData icon,