feat: relay ping with RTT display, fix dead_code warning

- New ping_relay Tauri command: QUIC connect with 3s timeout, returns RTT ms
- Relay status shown next to input field: "42ms" (green) or "offline" (red)
- Auto-pings on app startup and debounced on relay input change
- Fix SyncWrapper dead_code warning with #[allow(dead_code)]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/index.html

@@ -14,7 +14,10 @@
         <p class="subtitle">Encrypted Voice</p>
         <div class="form">
           <label>Relay
-            <input id="relay" type="text" value="193.180.213.68:4433" />
+            <div class="relay-row">
+              <input id="relay" type="text" value="193.180.213.68:4433" />
+              <span id="relay-status" class="relay-status"></span>
+            </div>
           </label>
           <label>Room
             <input id="room" type="text" value="android" />