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

frontend/src/i18n/locales/en/channels.json

@@ -46,6 +46,7 @@
   "cols": {
     "priority": "Prio",
     "channel": "Channel",
+    "about": "About",
     "stored": "Stored",
     "subs": "Subscribers",
     "lastUpload": "Last upload",

frontend/src/i18n/locales/hu/channels.json

@@ -46,6 +46,7 @@
   "cols": {
     "priority": "Prio",
     "channel": "Csatorna",
+    "about": "Leírás",
     "stored": "Tárolt",
     "subs": "Feliratkozók",
     "lastUpload": "Utolsó feltöltés",