feat(managers): About column with hover overlay on both channel tables (E4 S2)

- Backend: add channel description to the shared _channel_summary projection, so both
  the subscribed list and the discovery list carry it (no migration; description is
  already stored from the channels.list call)
- New AboutCell: one truncated line in the table + full text in a portalled, always-on
  glass hover/focus overlay (wider than Tooltip, flips above the anchor near the bottom,
  closes on scroll/resize)
- About column (after Channel) on both the subscribed and discovery tables; hidden in the
  mobile card fallback

backend/app/routes/channels.py

@@ -214,6 +214,7 @@ def _channel_summary(ch: Channel) -> dict:
         "thumbnail_url": ch.thumbnail_url,
         "subscriber_count": ch.subscriber_count,
         "video_count": ch.video_count,
+        "description": ch.description,
     }