feat(channels): dedicated channel page + ephemeral explore UI

Frontend for the channel-explore feature:
- ChannelPage: banner/avatar/stats header, Subscribe/unsubscribe, an "exploring" badge
  while browsing an un-subscribed channel, Videos/About tabs. Reuses Feed scoped to the
  channel (scope=all + source=all so the per-user ephemeral videos show). Auto-ingests
  recent uploads on first visit (background, with a loading note) + "Load more from
  YouTube" to page deeper; skipped for demo / already-subscribed channels.
- App: openChannel/closeChannel as a Back-aware sub-view (history.state._chan, mirrors the
  YT-search _yt pattern); ChannelPage takes over the content column, nav rail stays.
- ChannelLink/cards/player: the channel name now opens our channel page (onChannelFilter →
  onOpenChannel); the in-card "only this channel" filter button is dropped (the page
  subsumes it). PlayerModal channel-name wiring follows in the next commit.
- api: channelDetail + exploreChannel; ChannelDetail/ExploreResult types.
- i18n EN/HU/DE: channel namespace, explore_cleanup scheduler labels, explore config group,
  channels_explore quota label.

frontend/src/App.tsx

@@ -28,6 +28,7 @@ import Header from "./components/Header";
 import NavSidebar from "./components/NavSidebar";
 import Sidebar from "./components/Sidebar";
 import Feed from "./components/Feed";
+import ChannelPage from "./components/ChannelPage";
 import Channels, { type ChannelStatusFilter, type ChannelsView } from "./components/Channels";
 import Playlists from "./components/Playlists";
 import Stats from "./components/Stats";
@@ -140,6 +141,22 @@ export default function App() {
     if (window.history.state?._yt) window.history.back();
     else setYtSearch(null);
   }, []);
+  // The open channel page (null = no channel page). Like the YouTube-search sub-view it owns a
+  // browser-history entry (history.state._chan = channel id, _chanName = a display name to show
+  // before the detail loads), so Back closes the channel page (after any player opened over it)
+  // and reload returns to the normal app. Channels/videos are reached by id, so it's not in the URL.
+  const [channelView, setChannelView] = useState<{ id: string; name?: string } | null>(null);
+  const openChannel = useCallback((id: string, name?: string) => {
+    setChannelView({ id, name });
+    const st = window.history.state || {};
+    if (st._chan)
+      window.history.replaceState({ ...st, _chan: id, _chanName: name ?? null }, "");
+    else window.history.pushState({ ...st, _chan: id, _chanName: name ?? null }, "");
+  }, []);
+  const closeChannel = useCallback(() => {
+    if (window.history.state?._chan) window.history.back();
+    else setChannelView(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");
@@ -247,7 +264,8 @@ export default function App() {
   useEffect(() => {
     // Drop any stale _yt from a prior session (a reload starts on the normal feed; ytSearch
     // begins null), so the first Back doesn't resurrect a search we're no longer showing.
-    const { _yt: _staleYt, ...rest } = window.history.state || {};
+    const { _yt: _staleYt, _chan: _staleChan, _chanName: _staleChanName, ...rest } =
+      window.history.state || {};
     window.history.replaceState({ ...rest, sfPage: page }, "");
     function onPop(e: PopStateEvent) {
       const p = (e.state?.sfPage as Page) ?? "feed";
@@ -273,6 +291,9 @@ export default function App() {
       // The YouTube-search sub-view rides in history.state._yt, so Back/Forward restores or
       // clears it to match the entry we landed on (and a player popped first leaves it intact).
       setYtSearch((e.state?._yt as string) ?? null);
+      // The channel page rides in history.state._chan the same way.
+      const chan = e.state?._chan as string | undefined;
+      setChannelView(chan ? { id: chan, name: (e.state?._chanName as string) ?? undefined } : null);
     }
     window.addEventListener("popstate", onPop);
     return () => window.removeEventListener("popstate", onPop);
@@ -518,6 +539,18 @@ export default function App() {
         language={i18n.language as LangCode}
       />
       <div className="relative flex-1 min-w-0 flex flex-col">
+        {channelView ? (
+          <ChannelPage
+            key={channelView.id}
+            channelId={channelView.id}
+            initialName={channelView.name}
+            me={meQuery.data!}
+            view={view}
+            onBack={closeChannel}
+            onOpenChannel={openChannel}
+          />
+        ) : (
+          <>
         <Header
           me={meQuery.data!}
           filters={filters}
@@ -556,10 +589,7 @@ export default function App() {
               setView={setChannelsView}
               filtersResetToken={channelsFilterReset}
               onOpenWizard={() => setWizardOpen(true)}
-              onViewChannel={(id, name) => {
-                setFilters({ ...filters, channelId: id, channelName: name, show: "all" });
-                setPage("feed");
-              }}
+              onViewChannel={(id, name) => openChannel(id, name)}
               onFilterByTag={(tagId, name) => {
                 setFilters({ ...filters, tags: [tagId], channelId: undefined, channelName: undefined });
                 setPage("feed");
@@ -599,10 +629,13 @@ export default function App() {
               ytSearch={ytSearch}
               onYtSearch={enterYtSearch}
               onExitYtSearch={exitYtSearch}
+              onOpenChannel={openChannel}
             />
           )}
           </main>
         </div>
+          </>
+        )}
         {/* Toasts rise from the bottom-left, by the notification bell in the nav rail.
             Anchored inside the content column so they clear the sidebar automatically
             (collapsed or expanded) without tracking its width. */}