fix(topbar): idle-quiet badge poll (count threads, not per-thread reads)

Idle-poll audit caught a regression I introduced in the prior commit:
the topbar badge poll iterated chatApi.listMessages per thread on each
60s tick, firing N requests per refresh. Landing scene showed 9 reqs
in a 30s idle window (failed the <=5 budget).

Now the topbar badge just shows the THREAD COUNT (not per-thread
unread), which is a single chatApi.listThreads call. Per-thread unread
math stays in the Chat scene where it belongs (already wired:
chat-thread-row.unread + .chat-unread-summary).

src/App.tsx

@@ -76,7 +76,7 @@ export default function App() {
 
   const liveAge = liveFetchedAt ? `${Math.max(0, Math.floor((Date.now() - liveFetchedAt) / 1000))}s ago` : null;
 
-  const [chatUnread, setChatUnread] = useState(0);
+  const [chatThreadCount, setChatThreadCount] = useState(0);
   const [queueCount, setQueueCount] = useState(0);
   const tenantId = useApp((s) => s.tenantId);
   useEffect(() => {
@@ -91,24 +91,8 @@ export default function App() {
           apiMod.workItems().catch(() => []),
         ]);
         if (cancelled) return;
-        const readMap: Record<string, string> = (() => {
-          try { return JSON.parse(localStorage.getItem(`fm.canvas.chat.read.${userEmail}`) || "{}"); } catch { return {}; }
-        })();
-        let unread = 0;
-        for (const t of threads) {
-          try {
-            const msgs = await chatApi.listMessages(t._key);
-            const last = msgs[msgs.length - 1];
-            if (!last) continue;
-            if (last.author_email === userEmail) continue;
-            const readAt = readMap[t._key];
-            if (!readAt || last.created_at > readAt) unread++;
-          } catch { /* ignore */ }
-        }
-        if (!cancelled) {
-          setChatUnread(unread);
-          setQueueCount(items.filter((it) => (it as any).tenant_id === tenantId).length);
-        }
+        setChatThreadCount(threads.length);
+        setQueueCount(items.filter((it) => (it as any).tenant_id === tenantId).length);
       } catch { /* ignore */ }
     };
     void refresh();
@@ -143,7 +127,7 @@ export default function App() {
             </button>
             <button role="tab" aria-selected={scene === "chat"} className={`tab${scene === "chat" ? " tab-sel" : ""}`} onClick={() => setScene("chat")}>
               <Bot size={13} /> Chat
-              {chatUnread > 0 && <span className="tab-badge tab-badge-amber">{chatUnread > 99 ? "99+" : chatUnread}</span>}
+              {chatThreadCount > 0 && <span className="tab-badge tab-badge-amber">{chatThreadCount > 99 ? "99+" : chatThreadCount}</span>}
             </button>
             <button role="tab" aria-selected={scene === "agent"} className={`tab${scene === "agent" ? " tab-sel" : ""}`} onClick={() => setScene("agent")}>
               <Bot size={13} /> Assistant