feat(channels): a "Discover from your history" tab for the channels watched but not followed

The Channel manager lists the channels the Takeout history shows watched
often but not followed, the most watched first: twenty at a time, the
channel's About on hover, its name to its page, Follow, and Not interested
with an Undo. A row's count opens the videos it counts, ready to play one
or all. Before a history is uploaded it points to the upload; while the
import still asks YouTube it says more channels may join.

frontend/src/modules/feed/ChannelPage.tsx

@@ -157,6 +157,8 @@ export default function ChannelPage({
       qc.invalidateQueries({ queryKey: qk.channel(channelId) });
       qc.invalidateQueries({ queryKey: qk.feed() });
       qc.invalidateQueries({ queryKey: qk.blockedChannels() });
+      // A blocked channel is off the watched-channels list; unblocked, it may be back on it.
+      qc.invalidateQueries({ queryKey: qk.watchedChannels() });
     },
     // No onError: a local DB op (not a YouTube write, so no 403 "connect" case). The app-wide
     // reporter names the failure the error dialog does not cover (a 404 for a channel gone).