fix: WebSocket heartbeat to prevent idle drops + debounce reconnect sound

30s ping/pong keepalive prevents proxy/NAT from killing idle connections.
Reconnect sound now only plays if downtime exceeds 3s, suppressing phantom
audio from brief network blips.

packages/server/src/ws/handler.ts

@@ -604,6 +604,12 @@ export async function registerWebSocket(app: FastifyInstance): Promise<void> {
         return;
       }
 
+      // Fast-path heartbeat — never reaches business logic
+      if (parsed.type === 'ping') {
+        ws.send(JSON.stringify({ type: 'pong' }));
+        return;
+      }
+
       // Handle authenticated events
       if (userId && username) {
         handleClientEvent(parsed, userId, username);