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/VirtualFeed.tsx

@@ -40,7 +40,7 @@ export interface VirtualFeedProps {
   view: "grid" | "list";
   onState: (id: string, status: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
-  onChannelFilter: (channelId: string, channelName: string) => void;
+  onOpenChannel: (channelId: string, channelName: string) => void;
   onResetState: (id: string) => void;
   onOpen: (v: Video, startAt?: number | null) => void;
   hasNextPage: boolean;
@@ -53,7 +53,7 @@ export default function VirtualFeed({
   view,
   onState,
   onToggleSave,
-  onChannelFilter,
+  onOpenChannel,
   onResetState,
   onOpen,
   hasNextPage,
@@ -149,7 +149,7 @@ export default function VirtualFeed({
                     view="grid"
                     onState={onState}
                     onToggleSave={onToggleSave}
-                    onChannelFilter={onChannelFilter}
+                    onOpenChannel={onOpenChannel}
                     onResetState={onResetState}
                     onOpen={onOpen}
                   />
@@ -162,7 +162,7 @@ export default function VirtualFeed({
                   view="list"
                   onState={onState}
                   onToggleSave={onToggleSave}
-                  onChannelFilter={onChannelFilter}
+                  onOpenChannel={onOpenChannel}
                   onResetState={onResetState}
                   onOpen={onOpen}
                 />