fix(favorites): error handling + state hydration + input validation [#87 review]

- addFavorite/removeFavorite now wrap axios in try/catch: a 409 (already favorited,
  common on a stale heart) or 404 resyncs instead of throwing an unhandled promise
  rejection; other errors surface a toast.
- fetchHomeData refreshes favorites BEFORE the TTL cache-return (was appended after
  the early return, so warm-cache loads never refreshed); removed the now-redundant
  trailing call. App.vue onMounted also hydrates favorites so deep-links to Search/
  Playlist show correct hearts.
- favorites API: GET /check rejects non-string (array) query params with 400 instead
  of a 500; POST defaults req.body to {} so a missing JSON body yields the intended 400.

web/src/App.vue

@@ -143,6 +143,10 @@ function cycleMobileMode() {
 onMounted(async () => {
   playerStore.loadTheme();
   connect();
+  // Hydrate favorites once per session so deep-links / hard refreshes onto
+  // Search or Playlist render hearts correctly without first visiting Home.
+  // (fire-and-forget; fetchFavorites swallows the 401 when not yet logged in.)
+  playerStore.fetchFavorites();
   syncTimer = setInterval(() => playerStore.syncElapsed(), 3000);
   mobileRaf = requestAnimationFrame(updateMobileProgress);
   // Reconcile the dedicated-link scope only after the bot list is known: the