feat: add configurable close protection setting (#123)

- Add Close Protection toggle to Settings dialog
- Save setting to localStorage (default: enabled)
- Make beforeunload confirmation conditional
- Settings button now always visible in header
- Add shadcn Switch and Label components

package.json

@@ -28,9 +28,11 @@
         "@openrouter/ai-sdk-provider": "^1.2.3",
         "@opentelemetry/sdk-trace-node": "^2.2.0",
         "@radix-ui/react-dialog": "^1.1.6",
+        "@radix-ui/react-label": "^2.1.8",
         "@radix-ui/react-scroll-area": "^1.2.3",
         "@radix-ui/react-select": "^2.2.6",
         "@radix-ui/react-slot": "^1.1.2",
+        "@radix-ui/react-switch": "^1.2.6",
         "@radix-ui/react-tooltip": "^1.1.8",
         "@vercel/analytics": "^1.5.0",
         "@xmldom/xmldom": "^0.9.8",