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/components/Feed.tsx

@@ -77,6 +77,7 @@ export default function Feed({
   ytSearch,
   onYtSearch,
   onExitYtSearch,
+  onOpenChannel,
 }: {
   filters: FeedFilters;
   setFilters: (f: FeedFilters) => void;
@@ -91,6 +92,9 @@ export default function Feed({
   ytSearch: string | null;
   onYtSearch: (q: string) => void;
   onExitYtSearch: () => void;
+  // Open a channel's dedicated page (from a card's channel name / avatar). Threaded down to
+  // the cards via VirtualFeed.
+  onOpenChannel?: (channelId: string, channelName?: string) => void;
 }) {
   const { t } = useTranslation();
   const [overrides, setOverrides] = useState<Record<string, string>>({});
@@ -257,11 +261,11 @@ export default function Feed({
     [qc]
   );
 
-  const onChannelFilter = useCallback(
+  const handleOpenChannel = useCallback(
     (channelId: string, channelName: string) => {
-      setFilters({ ...filters, channelId, channelName });
+      onOpenChannel?.(channelId, channelName);
     },
-    [filters, setFilters]
+    [onOpenChannel]
   );
 
   const onToggleSave = useCallback(
@@ -347,7 +351,7 @@ export default function Feed({
               view={view}
               onState={onState}
               onToggleSave={onToggleSave}
-              onChannelFilter={onChannelFilter}
+              onOpenChannel={handleOpenChannel}
               onResetState={onResetState}
               onOpen={openVideo}
               hasNextPage={false}
@@ -568,7 +572,7 @@ export default function Feed({
           view={view}
           onState={onState}
           onToggleSave={onToggleSave}
-          onChannelFilter={onChannelFilter}
+          onOpenChannel={handleOpenChannel}
           onResetState={onResetState}
           onOpen={openVideo}
           hasNextPage={!!hasNextPage}