Fix stale-socket race in WS reconnect logic

React StrictMode double-invokes the effect that opens the chat WebSocket in
dev (mount -> cleanup -> mount again, on purpose, to catch exactly this
class of bug). The first socket gets abandoned in cleanup, but its onclose
still fires asynchronously afterward -- and unconditionally ran
`socketRef.current = null`, even after the second (real) socket had already
taken over. That silently orphaned a perfectly live connection: still open
and receiving broadcasts fine, but nothing left holding a reference to send
on, so outgoing messages/edits went nowhere with no visible error.

Found via manual testing: messages sent through the UI weren't appearing,
but a raw WebSocket opened by hand (bypassing React entirely) joined and
sent a message successfully, isolating the bug to the reconnect logic
rather than the backend.

Fix: each socket's onopen/onclose now checks it's still the one referenced
by socketRef before mutating shared state, so a stale/superseded socket's
events are inert instead of clobbering whatever socket is actually current.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/ws/useChatSocket.ts

@@ -29,16 +29,30 @@ export function useChatSocket({ roomId, onMessage, onUnauthenticated }: UseChatS
       socketRef.current = ws
 
       ws.onopen = () => {
+        // Guards against React StrictMode's dev-only double-invoke of this
+        // effect (mount -> cleanup -> mount again): the first socket gets
+        // abandoned in cleanup, but its own open/close events can still
+        // fire asynchronously afterward. Without this check, a stale
+        // socket's callbacks can stomp on state that the second (real)
+        // socket already owns.
+        if (socketRef.current !== ws) return
         reconnectDelay = RECONNECT_BASE_DELAY_MS
         setConnected(true)
         ws.send(JSON.stringify({ type: 'join', room_id: roomId }))
       }
 
       ws.onmessage = (event) => {
+        if (socketRef.current !== ws) return
         onMessageRef.current(JSON.parse(event.data) as ServerEnvelope)
       }
 
       ws.onclose = (event) => {
+        // Same guard as onopen -- a stale/abandoned socket's close event
+        // must not null out the reference to whatever socket has actually
+        // taken over since (this was a real bug: the abandoned socket's
+        // delayed onclose was silently orphaning a perfectly live
+        // connection, with nothing left referencing it to send on).
+        if (socketRef.current !== ws) return
         setConnected(false)
         socketRef.current = null