fix(frontend): move MessageBubble hooks above the early return

The three useMemo calls ran after the isUser early return, so they
were skipped entirely for user messages, violating React's rules of
hooks. Caught by the react-hooks/rules-of-hooks lint rule added last
commit. Pure reordering, no logic change; tsc and the full vitest
suite (38/38) still pass.

frontend/src/components/Chat/MessageBubble.tsx

@@ -103,6 +103,24 @@ function CopyMessageButton({ content }: { content: string }) {
 export function MessageBubble({ message, isLive = false }: Props) {
   const isUser = message.role === 'user';
 
+  const cleanContent = useMemo(() => stripThinkTags(message.content), [message.content]);
+
+  // Build a ref→source lookup once per render. Memoized so the rehype plugin
+  // identity stays stable until the source list actually changes.
+  const sourcesMap = useMemo(() => {
+    const m = new Map<number, NonNullable<ChatMessage['researchSources']>[number]>();
+    for (const s of message.researchSources ?? []) {
+      if (typeof s.ref === 'number') m.set(s.ref, s);
+    }
+    return m;
+  }, [message.researchSources]);
+
+  const rehypePlugins = useMemo(() => {
+    const base: any[] = [[rehypeHighlight, { detect: true }], rehypeKatex];
+    if (sourcesMap.size > 0) base.push([rehypeCitations, { sources: sourcesMap }]);
+    return base;
+  }, [sourcesMap]);
+
   if (isUser) {
     return (
       <div className="flex justify-end mb-4">
@@ -122,24 +140,6 @@ export function MessageBubble({ message, isLive = false }: Props) {
     );
   }
 
-  const cleanContent = useMemo(() => stripThinkTags(message.content), [message.content]);
-
-  // Build a ref→source lookup once per render. Memoized so the rehype plugin
-  // identity stays stable until the source list actually changes.
-  const sourcesMap = useMemo(() => {
-    const m = new Map<number, NonNullable<ChatMessage['researchSources']>[number]>();
-    for (const s of message.researchSources ?? []) {
-      if (typeof s.ref === 'number') m.set(s.ref, s);
-    }
-    return m;
-  }, [message.researchSources]);
-
-  const rehypePlugins = useMemo(() => {
-    const base: any[] = [[rehypeHighlight, { detect: true }], rehypeKatex];
-    if (sourcesMap.size > 0) base.push([rehypeCitations, { sources: sourcesMap }]);
-    return base;
-  }, [sourcesMap]);
-
   return (
     <div className="group mb-6">
       {/* Deep Research timeline (steps + status) */}