fix: resolve heartbeat race condition causing session stalls (#155)

This fixes a race condition where the client watchdog timeout (25s) was shorter than the server heartbeat interval (30s).

    Changes:
    - Server: Decreased heartbeat interval from 30s to 15s to send keepalives more frequently.
    - Client: Increased stale connection threshold from 25s to 45s to allow for 3 missed heartbeats before reconnecting.

    This ensures stable connections during long subagent tasks where no text generation occurs.

Co-authored-by: OpenCode Test <test@opencode.ai>

packages/ui/src/hooks/useEventStream.ts

@@ -1675,7 +1675,7 @@ export const useEventStream = () => {
       if (hasBusySessions) {
         void refreshSessionActivityStatus();
       }
-      if (now - lastEventTimestampRef.current > 25000) {
+      if (now - lastEventTimestampRef.current > 45000) {
         Promise.resolve().then(async () => {
           try {
             const healthy = await opencodeClient.checkHealth();