feat(design): a channel's and a Plex playlist's one action list (C1) — the row's "…", Delete + undo, an action toast's 8 s floor (E28 S3e2)

- Channels: the row's full-history, hide and unsubscribe buttons become its "…" at the row end
  (useChannelActions: open, pin, YouTube, copy name / link, full history or the admin reset,
  hide with an undo toast, unsubscribe behind its danger confirm); the card layout docks the "…"
  beside the name (ChannelLayoutGrid moreKey). The admin reset's confirm now names the reset.
- Plex playlist page: a title, a show group and a season each open the page's one row menu (a
  title's is its usePlexCardActions list plus the removal); no X at rest; removals take an undo
  toast whose Undo puts every row back at its own index in one request (bulk add `positions`);
  j/k walk the visible rows, Delete removes the walked one; rename and delete in the header's "…".
- notify: a toast with an action lives at least 8 s (the owner's call), whatever the toast setting.

frontend/src/components/ChannelDiscovery.tsx

@@ -93,8 +93,8 @@ export default function ChannelDiscovery({
   );
 
   const columns: Column<DiscoveredChannel>[] = [
-    // Actions first, mirroring the Subscriptions table (where it sits right after Prio) so the two
-    // tabs' action clusters line up in the same place.
+    // Subscribe first: it is this table's primary action, the one thing a discovery row is for (the
+    // Subscriptions table has no such button — its row ends in the channel's "…", E28 C1).
     {
       key: "actions",
       header: t("channels.cols.actions"),