feat: add auto-mark as read at 100% reading progress

- Add autoMarkAsReadAt100 setting (default: false)
- Add checkbox in Layout & Behavior settings
- Automatically mark article as read after 2 seconds at 100% progress
- Trigger same animation as manual mark as read button
- Move isNostrArticle computation earlier for useCallback deps
- Move handleMarkAsRead to useCallback for use in auto-mark effect

src/components/Settings/LayoutBehaviorSettings.tsx

@@ -130,6 +130,19 @@ const LayoutBehaviorSettings: React.FC<LayoutBehaviorSettingsProps> = ({ setting
           <span>Auto-scroll to last reading position</span>
         </label>
       </div>
+
+      <div className="setting-group">
+        <label htmlFor="autoMarkAsReadAt100" className="checkbox-label">
+          <input
+            id="autoMarkAsReadAt100"
+            type="checkbox"
+            checked={settings.autoMarkAsReadAt100 ?? false}
+            onChange={(e) => onUpdate({ autoMarkAsReadAt100: e.target.checked })}
+            className="setting-checkbox"
+          />
+          <span>Automatically mark as read when reading progress is 100%</span>
+        </label>
+      </div>
     </div>
   )
 }