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/i18n/locales/en/channel.json

@@ -0,0 +1,20 @@
+{
+  "back": "Back",
+  "exploringBadge": "Exploring",
+  "subscribe": "Subscribe",
+  "subscribedState": "Subscribed",
+  "unsubscribe": "Unsubscribe",
+  "unsubTitle": "Unsubscribe?",
+  "unsubBody": "Stop following {{name}} on YouTube?",
+  "subscribed": "Subscribed to {{name}}",
+  "subscribers": "{{formatted}} subscribers",
+  "videoCount_one": "{{count}} video",
+  "videoCount_other": "{{count}} videos",
+  "totalViews": "{{formatted}} views",
+  "joined": "Joined {{date}}",
+  "loadingVideos": "Loading this channel's videos…",
+  "tabVideos": "Videos",
+  "tabAbout": "About",
+  "loadMore": "Load more from YouTube",
+  "noDescription": "This channel has no description."
+}