feat(e10): open the app's menu with a right-click, starting with playlist rows

S2's headline was never a new Menu: `ActionMenu` already sits on `AnchoredPopover`
(arrows, typeahead, Escape, anchor-flip). What was missing is an INPUT PATH into it, so
this adds `useContextMenu` + `<ContextMenu>` over the same `ActionMenuItem[]` and the same
`MenuItemList` — a row cannot grow a right-click menu that looks or behaves unlike its own
⋯ menu. Three ways in: right-click anywhere on the row, long-press on touch/pen, and
Shift+F10 / the Menu key. The LEFT button is untouched and keeps the row's own action.

One popover per list, anchored to a virtual element at the pointer (native behaviour, and
a table of 200 rows would otherwise mount 200 floating contexts to keep 199 shut).

Platform bits it needed: `hoverAlso` on AnchoredPopover (a submenu falls open under a
mouse and is tapped by a finger — mouse-only, so a tap's synthetic mouseenter can't race
the click toggle) and `elementRef` on PopoverItem (the submenu anchors to the very
menuitem that opens it, where aria-haspopup/expanded belong). `ActionMenuItem` becomes a
discriminated union — a row either fires or opens a submenu — plus `danger` and
`separatorBefore`.

First consumers, both in Playlists:
- item rows: play · move to another list (a submenu of every other playlist, one gesture)
  · to top / up / down / bottom · remove. Reordering goes through the same undoable store
  as the drag handle and is disabled under a view for the same reason, with the same
  explanation rather than silence.
- rail rows: rename (in the row that was right-clicked, keeping its cover so the rail
  doesn't jump) · delete. Both now run through `usePlaylistActions`, which also takes over
  the detail header's pencil and trash — the delete dialog's YouTube branch and the
  "what gets selected next" rule exist once, not twice.
- the rail gains a "recently changed" sort key off the new `updated_at`. NOT the default:
  a rail that reshuffles itself while you look at it costs more than it saves.

frontend/src/components/ui/ContextMenu.tsx

@@ -0,0 +1,26 @@
+import { MENU_SURFACE, MenuItemList, type ActionMenuItem } from "./ActionMenu";
+import { PopoverSurface } from "./AnchoredPopover";
+import { type ContextMenuController } from "./useContextMenu";
+
+/** The menu surface for a `useContextMenu` controller. Render ONE per list, outside the rows. */
+export function ContextMenu<T>({
+  menu,
+  ariaLabel,
+  items,
+}: {
+  menu: ContextMenuController<T>;
+  /** Accessible name for the panel — a context menu has no visible trigger to take one from. */
+  ariaLabel: string;
+  items: readonly ActionMenuItem[];
+}) {
+  return (
+    <PopoverSurface
+      popover={menu.popover}
+      ariaLabel={ariaLabel}
+      className={`${MENU_SURFACE} overflow-y-auto`}
+      style={{ maxHeight: "min(28rem, var(--fit-max-h))" }}
+    >
+      <MenuItemList items={items} onClose={menu.close} />
+    </PopoverSurface>
+  );
+}