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/lib/api.ts

@@ -76,6 +76,37 @@ export interface VideoDetail {
   duration_seconds: number | null;
 }
 
+export interface ChannelLink {
+  title: string | null;
+  url: string;
+}
+
+export interface ChannelDetail {
+  id: string;
+  title: string | null;
+  handle: string | null;
+  description: string | null;
+  thumbnail_url: string | null;
+  banner_url: string | null;
+  subscriber_count: number | null;
+  video_count: number | null;
+  total_view_count: number | null;
+  published_at: string | null;
+  external_links: ChannelLink[];
+  country: string | null;
+  subscribed: boolean;
+  explored: boolean;
+  stored_videos: number;
+  from_explore: boolean;
+  details_synced: boolean;
+}
+
+export interface ExploreResult {
+  channel_id: string;
+  ingested: number;
+  next_page_token: string | null;
+}
+
 export interface FeedResponse {
   items: Video[];
   has_more: boolean;
@@ -628,6 +659,21 @@ export const api = {
   subscribeChannel: (id: string) =>
     req(`/api/channels/${id}/subscribe`, { method: "POST" }),
   syncSubscriptions: () => req("/api/sync/subscriptions", { method: "POST" }),
+  // --- channel page (About detail + ephemeral exploration of un-subscribed channels) ---
+  channelDetail: (id: string): Promise<ChannelDetail> => req(`/api/channels/${id}`),
+  // Open an un-subscribed channel for browsing: ingest a page of its uploads (newest-first,
+  // marked via_explore) + record the exploration. `pageToken` continues into older uploads
+  // ("load more"). `quiet` so a quota-reserve 429 / load error renders inline on the page.
+  exploreChannel: (id: string, pageToken?: string | null): Promise<ExploreResult> => {
+    const p = new URLSearchParams();
+    if (pageToken) p.set("page_token", pageToken);
+    const qs = p.toString();
+    return req(
+      `/api/channels/${id}/explore${qs ? `?${qs}` : ""}`,
+      { method: "POST" },
+      { quiet: true }
+    );
+  },
 
   // --- playlists ---
   playlists: (containsVideoId?: string): Promise<Playlist[]> =>