fix(canvas/mobile): remove ?? [] from Zustand selector to prevent infinite render loop

React error #185 (Maximum update depth exceeded) on mobile chat tab.

Root cause: useCanvasStore((s) => s.agentMessages[agentId] ?? []) used
a `?? []` fallback in the selector. Zustand uses Object.is for selector
equality. When agentMessages[agentId] is undefined (initial state), the
fallback creates a NEW [] reference on every store update. Zustand sees
this as a state change and re-renders the component. The component reads
from the store again, gets another new [] reference, and the cycle
repeats until React hits the depth cap.

Fix: remove `?? []` from the selector (returns undefined when no messages)
and move the fallback to the useState initializer:
  storedMessages = useCanvasStore(selector)     // returns undefined | T[]
  [messages] = useState(() => (storedMessages ?? []).map(...))

The useState initializer only runs once on mount, so the `?? []`
there is safe — it creates the initial state once, then messages are
managed via setMessages.

Fixes issue #651.

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

canvas/src/components/mobile/MobileChat.tsx

@@ -54,9 +54,14 @@ export function MobileChat({
   // user sees their prior thread on entry. The store is updated by the
   // socket → ChatTab flows the desktop runs; on mobile we read from the
   // same buffer to keep state coherent across viewports.
-  const storedMessages = useCanvasStore((s) => s.agentMessages[agentId] ?? []);
+  // NOTE: do NOT use `?? []` in the selector — Zustand uses Object.is
+  // for selector equality. A fallback `?? []` creates a new [] reference on
+  // every store update when agentMessages[agentId] is undefined, causing an
+  // infinite re-render loop (React error #185 / Maximum update depth
+  // exceeded). The undefined case is handled by the initializer below.
+  const storedMessages = useCanvasStore((s) => s.agentMessages[agentId]);
   const [messages, setMessages] = useState<ChatMessage[]>(() =>
-    storedMessages.map((m) => ({
+    (storedMessages ?? []).map((m) => ({
       id: m.id,
       role: "agent",
       text: m.content,