feat: runtime login screen — remove token from build

- Add LoginScreen component with Gateway URL + Token fields
- Store credentials in localStorage (not in bundle)
- Auto-reconnect with stored credentials on reload
- Add logout button (LogOut icon) in Header
- Remove VITE_GATEWAY_TOKEN from .env.example
- VITE_GATEWAY_WS_URL now only pre-fills the URL field
- Dark neon theme consistent with rest of app

Closes feedback item #4

.env.example

@@ -1,2 +1,3 @@
+# Optional: pre-fill the Gateway URL field on the login screen
+# If not set, defaults to ws://<current-hostname>:18789
 VITE_GATEWAY_WS_URL=ws://localhost:18789
-VITE_GATEWAY_TOKEN=your-gateway-token-here