fix(webui): add getServerSnapshot to running-threads store for SSR

useLocalRunningThreadIds is now mounted on a force-dynamic route, so
useSyncExternalStore needs a getServerSnapshot to avoid React's
"Missing getServerSnapshot" error during server rendering. Use a
module-level EMPTY_IDS constant to keep snapshot identity stable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/hooks/useThreads.ts

@@ -99,6 +99,7 @@ export function useBusyThreadIds(): ReadonlySet<string> {
 const runningThreadIds = new Set<string>();
 const runningListeners = new Set<() => void>();
 let runningSnapshot: ReadonlySet<string> = runningThreadIds;
+const EMPTY_IDS: ReadonlySet<string> = new Set();
 
 export function setThreadRunning(threadId: string, running: boolean): void {
   const changed = running
@@ -122,7 +123,11 @@ export function getRunningThreadIds(): ReadonlySet<string> {
 }
 
 export function useLocalRunningThreadIds(): ReadonlySet<string> {
-  return useSyncExternalStore(subscribeRunningThreads, getRunningThreadIds);
+  return useSyncExternalStore(
+    subscribeRunningThreads,
+    getRunningThreadIds,
+    () => EMPTY_IDS
+  );
 }
 
 /** Merged view of "this thread has an active run": the zero-latency local