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/components/Channels.tsx

@@ -27,6 +27,7 @@ import Tooltip from "./Tooltip";
 import DataTable, { type Column } from "./DataTable";
 import { PageToolbar } from "./PageShell";
 import ChannelLink from "./ChannelLink";
+import AboutCell from "./AboutCell";
 import ChannelDiscovery from "./ChannelDiscovery";
 import TagManager from "./TagManager";
 import { useConfirm } from "./ConfirmProvider";
@@ -301,6 +302,12 @@ export default function Channels() {
         />
       ),
     },
+    {
+      key: "about",
+      header: t("channels.cols.about"),
+      hideInCard: true,
+      render: (c) => <AboutCell text={c.description} />,
+    },
     {
       key: "stored",
       header: t("channels.cols.stored"),