Remove manual reveal button and fix vote sync on session transitions

Remove the "Reveal Votes" button since auto-reveal handles this. Add
poker:sync event so PokerRoom fetches current vote state on mount,
fixing checkmarks not appearing for other users on subsequent rounds.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/index.js

@@ -256,6 +256,17 @@ io.on('connection', (socket) => {
     }
   });
 
+  throttled('poker:sync', async ({ sessionId }) => {
+    try {
+      if (!sessionId) return;
+      const snapshot = await getSessionSnapshot(sessionId, socket.user.jiraCloudId);
+      if (!snapshot) return;
+      await emitSessionState(snapshot.session.roomId, sessionId, socket.user.jiraCloudId);
+    } catch (error) {
+      console.error('[socket] poker:sync failed:', error);
+    }
+  });
+
   throttled('poker:vote', async ({ sessionId, vote }) => {
     try {
       if (!sessionId) {