feat(channels): an Added column, and About cards that wait for a resting pointer

The channel manager lists when each follow reached the list (the
subscription import that first saw it, or a subscribe made here) as a
sortable Added date after Last upload. The About cell's card opens after the
pointer rests on it for half a second instead of at once, and a scroll puts
an open one away.

frontend/src/modules/channels/AboutCell.tsx

@@ -19,6 +19,9 @@ import { SURFACE } from "../../components/ui/surfaces";
  * The card keeps the platform's default hover role (`tooltip`); making this rich, interactive panel
  * properly keyboard-reachable belongs to the focus/a11y work, not to the mechanical migration.
  */
+// How long the pointer rests on a cell before its card opens.
+const ABOUT_REST_MS = 500;
+
 export default function AboutCell({ text, channelId }: { text: string | null; channelId: string }) {
   const preview = (text ?? "").trim();
   const [open, setOpen] = useState(false);
@@ -28,8 +31,11 @@ export default function AboutCell({ text, channelId }: { text: string | null; ch
     onOpenChange: setOpen,
     mode: "hover",
     offset: 6,
-    // Pop instantly like the old cell did; the close grace lets the cursor reach the card.
-    hoverDelay: { open: 0, close: 160 },
+    // Opens once the pointer rests on the cell, not as it crosses one: scrolling the table under a
+    // still pointer, or moving down the column, used to pop card after card. The close grace lets
+    // the cursor reach the card; a scroll puts an open one away.
+    hoverDelay: { open: ABOUT_REST_MS, close: 160 },
+    dismissOnScroll: true,
     // Cap the panel to the space between the trigger and the viewport edge (publishes --fit-max-h) so a
     // long About never runs its top off-screen — at high browser zoom the short viewport made a fixed
     // max-height overflow above the top edge, where the scroll container can't reach.