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/VideoCard.tsx

@@ -6,7 +6,6 @@ import {
   CheckCheck,
   Eye,
   EyeOff,
-  ListFilter,
   Play,
   RotateCcw,
 } from "lucide-react";
@@ -21,13 +20,11 @@ function Actions({
   onState,
   onResetState,
   onToggleSave,
-  onChannelFilter,
 }: {
   video: Video;
   onState: (id: string, status: string) => void;
   onResetState?: (id: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
-  onChannelFilter?: (channelId: string, channelName: string) => void;
 }) {
   const { t } = useTranslation();
   const act = (status: string) => (e: React.MouseEvent) => {
@@ -97,19 +94,6 @@ function Actions({
           <RotateCcw className="w-4 h-4" />
         </button>
       )}
-      {onChannelFilter && (
-        <button
-          onClick={(e) => {
-            e.preventDefault();
-            e.stopPropagation();
-            onChannelFilter(video.channel_id, video.channel_title ?? t("card.thisChannel"));
-          }}
-          title={t("card.onlyThisChannel")}
-          className="p-1.5 rounded-md hover:bg-surface text-muted hover:text-fg"
-        >
-          <ListFilter className="w-4 h-4" />
-        </button>
-      )}
     </div>
   );
 }
@@ -244,7 +228,7 @@ function VideoCard({
   onState,
   onResetState,
   onToggleSave,
-  onChannelFilter,
+  onOpenChannel,
   onOpen,
 }: {
   video: Video;
@@ -252,7 +236,7 @@ function VideoCard({
   onState: (id: string, status: string) => void;
   onResetState?: (id: string) => void;
   onToggleSave: (id: string, saved: boolean) => void;
-  onChannelFilter?: (channelId: string, channelName: string) => void;
+  onOpenChannel?: (channelId: string, channelName: string) => void;
   onOpen?: (v: Video, startAt?: number | null) => void;
 }) {
   const { t, i18n } = useTranslation();
@@ -303,18 +287,18 @@ function VideoCard({
         <Thumb video={video} className="w-44 aspect-video shrink-0" onOpen={onOpen} />
         <div className="min-w-0 flex-1">
           {title}
-          <a
-            href={video.channel_url}
-            target="_blank"
-            rel="noreferrer"
-            onClick={(e) => e.stopPropagation()}
-            className="text-sm text-muted truncate mt-0.5 block w-fit max-w-full hover:text-fg"
+          <button
+            onClick={(e) => {
+              e.stopPropagation();
+              onOpenChannel?.(video.channel_id, video.channel_title ?? t("card.thisChannel"));
+            }}
+            className="text-sm text-muted truncate mt-0.5 block w-fit max-w-full text-left hover:text-fg"
           >
             {video.channel_title}
-          </a>
+          </button>
           <div className="text-xs text-muted mt-0.5">{meta}</div>
         </div>
-        <Actions video={video} onState={onState} onResetState={onResetState} onToggleSave={onToggleSave} onChannelFilter={onChannelFilter} />
+        <Actions video={video} onState={onState} onResetState={onResetState} onToggleSave={onToggleSave} />
       </div>
     );
   }
@@ -335,17 +319,17 @@ function VideoCard({
         />
         <div className="min-w-0 flex-1">
           {title}
-          <a
-            href={video.channel_url}
-            target="_blank"
-            rel="noreferrer"
-            onClick={(e) => e.stopPropagation()}
-            className="text-sm text-muted truncate mt-0.5 block w-fit max-w-full hover:text-fg"
+          <button
+            onClick={(e) => {
+              e.stopPropagation();
+              onOpenChannel?.(video.channel_id, video.channel_title ?? t("card.thisChannel"));
+            }}
+            className="text-sm text-muted truncate mt-0.5 block w-fit max-w-full text-left hover:text-fg"
           >
             {video.channel_title}
-          </a>
+          </button>
           <div className="text-xs text-muted mt-0.5">{meta}</div>
-          <Actions video={video} onState={onState} onResetState={onResetState} onToggleSave={onToggleSave} onChannelFilter={onChannelFilter} />
+          <Actions video={video} onState={onState} onResetState={onResetState} onToggleSave={onToggleSave} />
         </div>
       </div>
     </div>