Studio: stop ActiveThreadSync clearing persisted draft on reload

ActiveThreadSync was reacting to aui's mainThreadId === null on mount
(aui hasn't booted yet) by calling setActiveThreadId(null), which
wiped the just-restored persisted draft id from localStorage and
emptied the doc panel for the user's thread. The previous fix only
covered the 'aui minted a different LOCALID' branch; it missed the
'mainThreadId is null while aui boots' branch.

Treat a null mainThreadId as a no-op for the sync. Explicit clears
(new chat, sidebar delete) keep going through setActiveThreadId(null)
directly, so this guard doesn't trap stale state — it just gives the
persisted draft id a chance to survive until aui finishes booting.

studio/frontend/src/features/chat/runtime-provider.tsx

@@ -1017,6 +1017,15 @@ function ActiveThreadSync({
     if (!enabled) {
       return;
     }
+    // Don't clobber a persisted activeThreadId while aui is still booting
+    // up. mainThreadId starts null on every page load before aui's async
+    // thread init finishes; without this guard we'd reset our persisted
+    // draft id to null and lose the doc chip mapping for that thread.
+    // Explicit clears (new chat / thread delete) go through
+    // setActiveThreadId(null) directly and bypass this effect.
+    if (!mainThreadId) {
+      return;
+    }
     const persisted = useChatRuntimeStore.getState().activeThreadId;
     // On page reload aui mints a fresh `__LOCALID_*` for its draft thread.
     // If we already have a persisted draft id, keep it so any RAG docs
@@ -1024,15 +1033,14 @@ function ActiveThreadSync({
     // chat history for the new mainThreadId starts empty either way, so
     // this only affects what we treat as the active id for doc/RAG lookup.
     if (
-      mainThreadId
+      persisted
-      && persisted
       && persisted !== mainThreadId
       && persisted.startsWith("__LOCALID_")
       && mainThreadId.startsWith("__LOCALID_")
     ) {
       return;
     }
-    setActiveThreadId(mainThreadId ?? null);
+    setActiveThreadId(mainThreadId);
   }, [enabled, mainThreadId, setActiveThreadId]);
 
   return null;