Grow the companion, fix idle fallback, add update banner

Companion: bump the corner mascot ~12% (128→144) and make animation
resolution robust — fall back to the idle clip when a mood has no clip of
its own, and pin the always-asleep cat to one pose so its Lottie never
reloads. This stops a bare emoji from replacing the cute companion on idle.

Update notifications: serve a build id at /api/version (a hash of the
embedded dist/index.html, which changes on every frontend deploy). The
client records its load-time id, polls every 90s (and on tab focus), and
floats a Mandarin-first "new version available" banner offering a refresh
when the id changes.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/App.tsx

@@ -7,8 +7,11 @@ import { DocList } from './components/DocList/DocList'
 import { EditorCore, type EditorChange } from './components/Editor/EditorCore'
 import { StatusBar } from './components/StatusBar/StatusBar'
 import { PetalCompanion } from './components/Companion/PetalCompanion'
+import { UpdateBanner } from './components/UpdateBanner/UpdateBanner'
+import { useVersionWatch } from './hooks/useVersionWatch'
 
 export default function App() {
+  const updateAvailable = useVersionWatch()
   const [docs, setDocs] = useState<DocSummary[]>([])
   const [currentDoc, setCurrentDoc] = useState<Document | null>(null)
   const [title, setTitle] = useState('')
@@ -250,6 +253,8 @@ export default function App() {
         </main>
       </div>
 
+      {updateAvailable && <UpdateBanner />}
+
       <PetalCompanion
         wordCount={wordCount}
         saveStatus={status}