fix: add socket reconnection backoff and clean up leaked listeners

[Flegma]

Summary

• Socket.ts: Replace fixed 1-second reconnect with exponential backoff (1s→30s) with jitter, max 50 retries. Clean up old WebSocket on reconnect to prevent listener accumulation.
• ChatLobby.vue: Stop previous lobbyListener before creating new one in lobbyId watcher
• MatchmakingConfirm.vue: Clear existing interval before creating new countdown, fix beforeUnmount null check

Closes 5stackgg/5stack-panel#386, 5stackgg/5stack-panel#387

Test plan

• Verify WebSocket reconnects with increasing delays when server is unavailable
• Verify reconnection resets to 1s delay after successful connection
• Verify switching chat lobbies doesn't accumulate listeners
• Verify matchmaking countdown works correctly on rapid confirmation changes

[lukepolo]

why have a max?