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/index.css

@@ -239,6 +239,15 @@ button, a, input {
   100% { opacity: 0; transform: translateY(-14px) scale(1.1); }
 }
 
+/* "New version available" banner — drifts down from the top like a petal. */
+.petal-update {
+  animation: petal-drop 360ms cubic-bezier(0.2, 0.8, 0.3, 1.2) both;
+}
+@keyframes petal-drop {
+  from { opacity: 0; transform: translateY(-14px) scale(0.96); }
+  to   { opacity: 1; transform: translateY(0) scale(1); }
+}
+
 /* Blinking caret in the Ask Petal bubble while awaiting the first token. */
 .petal-chat-caret {
   animation: petal-breathe 1s ease-in-out infinite;