feat(e24): pin to the comfort shelf from every card and row

S3 wires the shelf's editor. The gesture is the E10 right-click menu, so the
decision "this is comfort media" can be made where the thing is — the feed, a
Plex grid, a channel row, a playlist — rather than only on the page that lists
the shelf.

One shared piece, `components/comfortPin.tsx`: the five pin-kind constructors,
the shelf query, and ONE `ActionMenuItem` that switches Pin/Unpin honestly (and
says "not known yet" with a disabled row until the shelf has answered, since a
confident "Pin" over something already pinned is exactly the quiet lie the house
rules forbid). Surfaces that already have a row menu compose the item in
(`Playlists`, `PlaylistsRail`); the rest mount `ComfortPinMenu`.

Two structural notes:

- The feed's menu lives in `VirtualFeed`, not in `Feed` — that is the list, and
  it is also what gives the channel page the gesture for free, since it renders
  `<Feed channelScoped>` all the way down.
- Cards take `menuProps` as a FUNCTION of the row, not the built props object.
  `triggerProps` is stable across renders while its return value is not, and a
  fresh object prop would blow past `VideoCard`'s memo for every card in the
  virtualized feed on every parent render.

`DataTable` and `ChannelLayoutGrid` grow a generic `rowProps` so a channel row
answers the same gesture in both of the manager's layouts; the table stays
domain-free and never learns what a menu is.

On Home the shelf tile's menu is the whole editor: four move rows (front / left
/ right / end, reusing the playlist menu's ordering idiom) plus the unpin, over
menu rows rather than drag & drop — dragging inside a horizontal, scrolling
Carousel fights the scroll, and menu rows are keyboard-reachable for free. The
reorder is optimistic on the cached payload and re-asks the server on failure
rather than showing an order that was never accepted.

The exhausted shelf tile (a pinned channel with nothing unwatched left) is
`aria-disabled`, not `disabled`: measured in Chromium, a disabled button still
receives contextmenu and pointerdown, but it loses focus — and with it Shift+F10,
the only keyboard route to the menu it most needs.

Verified end to end in the browser against the live account: all five pin kinds,
the reorder (persisted across a reload), the unpin, and the honest Unpin label on
a resume tile that is already on the shelf.

frontend/src/components/PlexDiscover.tsx

@@ -10,6 +10,8 @@ import { toggleWatchState } from "../lib/plexWatch";
 import { usePlex } from "./PlexProvider";
 import { Carousel } from "./ui/Carousel";
 import { PlexPosterCard } from "./plexCards";
+import { PlexPinMenu } from "./ComfortPinMenu";
+import { useContextMenu } from "./ui/useContextMenu";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 
 // The Plex "Discover" landing (E5 S4): a stack of horizontal Carousel rails served from ONE
@@ -40,6 +42,8 @@ function CardRail({
   onToggleWatched: (c: PlexCard) => void;
   action?: ReactNode; // optional header control (e.g. the Surprise-me reshuffle button)
 }) {
+  // One comfort-pin menu per rail (see ui/ContextMenu), not one per poster.
+  const pinMenu = useContextMenu<PlexCard>();
   const heading =
     action != null ? (
       <span className="inline-flex items-center gap-2">
@@ -59,10 +63,12 @@ function CardRail({
               onOpen={() => onOpenCard(c)}
               onInfo={() => onInfoCard(c)}
               onToggleWatched={onToggleWatched}
+              menuProps={pinMenu.triggerProps}
             />
           </div>
         ))}
       </Carousel>
+      <PlexPinMenu menu={pinMenu} />
     </section>
   );
 }