feat(search): live YouTube search UI

Surface live YouTube search in the existing feed, triggered explicitly so the
expensive API call is never per-keystroke.

- Header: the search box still filters the local catalog as you type; Enter or a
  YouTube button escalates the term to a live search (hidden for the demo account).
- Feed: a dedicated infinite query renders results in the same VirtualFeed cards +
  in-app player, under a banner with a back button and a quota note. No auto-paginate
  (each page spends 100 units) — an explicit 'Load more (uses quota)' button instead;
  quota/limit errors (incl. 429) shown inline. The empty local feed offers a
  'Search YouTube for <q>' CTA.
- Library: a 'Search-discovered' toggle reveals search-ingested videos (hidden by
  default); sent as exclude_search_discovered.
- Admin: search_daily_limit_per_user config field; new videos_search quota label.
- All new strings translated in HU/EN/DE.

frontend/src/App.tsx

@@ -122,6 +122,9 @@ export default function App() {
   const saveMsgTimer = useRef<number | undefined>(undefined);
   const [sidebarLayout, setSidebarLayoutState] = useState<SidebarLayout>(loadLayout);
   const [page, setPageState] = useState<Page>(loadInitialPage);
+  // Live YouTube search term (null = normal feed). Ephemeral: not persisted and not in the URL,
+  // so a reload returns to the normal feed (a search spends quota, so we never auto-replay it).
+  const [ytSearch, setYtSearch] = useState<string | null>(null);
   const [wizardOpen, setWizardOpen] = useState(false);
   // Channel status chip, persisted so a reload (F5) keeps it; clamp a stale value at render.
   const [channelFilterRaw, setChannelFilter] = usePersistedState(LS.channelFilter, "all");
@@ -153,6 +156,8 @@ export default function App() {
   };
   useEffect(() => {
     if (page !== "channels") setFocusChannelName(null);
+    // Leaving the feed ends any live YouTube search, so coming back shows the normal feed.
+    if (page !== "feed") setYtSearch(null);
   }, [page]);
 
   function openReleaseNotes(highlight?: string) {
@@ -500,6 +505,7 @@ export default function App() {
           filters={filters}
           setFilters={setFilters}
           page={page}
+          onYtSearch={setYtSearch}
           onGoToFullHistory={() => {
             setChannelFilter("needs_full");
             setChannelsView("subscribed"); // the status filter applies to the subscriptions tab
@@ -572,6 +578,8 @@ export default function App() {
               canRead={meQuery.data!.can_read}
               isDemo={meQuery.data!.is_demo}
               onOpenWizard={() => setWizardOpen(true)}
+              ytSearch={ytSearch}
+              setYtSearch={setYtSearch}
             />
           )}
           </main>