fix: feedback round 2 — language, subscriptions, feed scope, UX

- Language detection: classify one cleaned+concatenated blob (strip emoji,
  @mentions, #tags, numbers, punctuation); fixes caps/emoji-heavy channels
  (e.g. Nessaj -> Hungarian, no more bogus Chinese/Korean)
- Feed now joins the user's subscriptions, so unsubscribing on YouTube removes a
  channel from the feed; periodic subscription re-sync job picks up changes
- Watched/Saved/Hidden views ignore the Shorts/live default-hiding so the full
  set is visible (fixes hidden videos missing from the Hidden view)
- Persist feed filters + search across reloads (localStorage)
- 3D polish: cards lift with shadow on hover; chips/buttons get depth and a
  press effect; undo toast lasts longer

frontend/src/App.tsx

@@ -24,11 +24,26 @@ const DEFAULT_FILTERS: FeedFilters = {
   show: "unwatched",
 };
 
+const FILTERS_KEY = "subfeed.filters";
+
+function loadFilters(): FeedFilters {
+  try {
+    return { ...DEFAULT_FILTERS, ...JSON.parse(localStorage.getItem(FILTERS_KEY) || "{}") };
+  } catch {
+    return DEFAULT_FILTERS;
+  }
+}
+
 export default function App() {
   const [theme, setThemeState] = useState<ThemePrefs>(() => loadLocalTheme());
-  const [filters, setFilters] = useState<FeedFilters>(DEFAULT_FILTERS);
+  const [filters, setFiltersState] = useState<FeedFilters>(loadFilters);
   const [view, setView] = useState<"grid" | "list">("grid");
 
+  function setFilters(next: FeedFilters) {
+    setFiltersState(next);
+    localStorage.setItem(FILTERS_KEY, JSON.stringify(next));
+  }
+
   useEffect(() => applyTheme(theme), [theme]);
 
   const meQuery = useQuery({ queryKey: ["me"], queryFn: api.me });