research canvas: move the setTopic(null) reset inside the async load

Second lint pass caught the standalone setTopic(null) in the early-return
branch of the effect — still synchronous at the top of the useEffect. Fold
that guard into the async load() so every setState landing on this hook
runs inside the async wrapper.

frontend/src/components/dashboard/ResearchCanvas.tsx

@@ -60,12 +60,12 @@ export function ResearchCanvas({
   const [acting, setActing] = useState(false);
 
   useEffect(() => {
-    if (!selectedId) {
-      setTopic(null);
-      return;
-    }
     let alive = true;
     const load = async () => {
+      if (!selectedId) {
+        if (alive) setTopic(null);
+        return;
+      }
       setLoading(true);
       setError(null);
       try {