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/src-tauri/src/engine.rs

@@ -18,6 +18,7 @@ const FRAME_SAMPLES: usize = 960;
 /// Wrapper to make non-Sync audio handles safe to store in shared state.
 /// The audio handle is only accessed from the thread that created it (drop),
 /// never shared across threads — Sync is safe.
+#[allow(dead_code)]
 struct SyncWrapper(Box<dyn std::any::Any + Send>);
 unsafe impl Sync for SyncWrapper {}