fix(player): a related video played is tracked, and its list is honest about its state

A related video played in place is the player's item now: its title and its own watched /
playlist / download controls, its progress checkpointed and resumed, marked watched at its end —
and its end does not step the queue. Back returns to the opened video, where it was left.

The related list:
- its marks announce through the shared notice (the Notification Center finds them), join the
  host's player session (the feed refetches on close), and stay per write, so an earlier answer no
  longer undoes a later mark still on its way;
- every host's state write patches the cached related lists, which are never refetched;
- kept for the session (gcTime), a retry asks for the page that failed, a list read so far with
  nothing unseen says so, a refusal a retry cannot answer offers none, and the followed mark names
  itself on hover.

Turned off on the instance (a limit of 0), the tab, the drawer button and the live-search buttons
are left out.

Keys: the player's keys and its focus reclaim defer to a layer above it (the drawer); going wide
moves the drawer's list into the panel's tab; `j` goes on from the slot of a row removed inside the
top layer; the help sheet lists the list keys over a layer that holds a list.

frontend/src/lib/keymap.ts

@@ -472,22 +472,33 @@ export function useActiveScopes(): KeyScope[] {
 // One sheet for the whole app, opened by the global `h` (or a surface's own keyboard button), so
 // there is exactly one answer to "what can I press here" instead of one per player.
 const helpOpen = createStore(false);
-// Whether the sheet was opened OVER a layer that owns the keyboard (a player, a dialog). The page-level
-// keys are quiet there, so the sheet must not list them — and it cannot ask the layer stack itself
-// once it is showing, because its own Modal is a layer too. So the answer is taken at the moment of
-// opening, before that Modal mounts. (The palette's "Help" row runs only after the palette has closed.)
-const helpOverLayer = createStore(false);
+// What the sheet was opened OVER: the page, a layer that owns the keyboard (a player, a dialog), or
+// such a layer with a list inside it (the player's related videos). The page-level keys are quiet over
+// a layer, so the sheet must not list them — except the list keys, which walk a list inside the
+// topmost layer (see `dispatch`'s `inTopLayer`). The sheet cannot ask the layer stack itself once it
+// is showing, because its own Modal is a layer too, so the answer is taken at the moment of opening,
+// before that Modal mounts. (The palette's "Help" row runs only after the palette has closed.)
+export type HelpOver = "page" | "layer" | "layerWithList";
+const helpOverLayer = createStore<HelpOver>("page");
+
+// Whether the topmost layer holds a list the keyboard walks. The lists are listNav's, which imports
+// this module and so cannot be imported back: it hands the question in when it loads.
+let listInTopLayer: () => boolean = () => false;
+export const setListInTopLayerProbe = (probe: () => boolean): void => {
+  listInTopLayer = probe;
+};
 
 export const closeKeyboardHelp = (): void => helpOpen.set(false);
 export const toggleKeyboardHelp = (): void => {
   const next = !helpOpen.get();
-  if (next) helpOverLayer.set(anyBlockingLayer());
+  if (next)
+    helpOverLayer.set(!anyBlockingLayer() ? "page" : listInTopLayer() ? "layerWithList" : "layer");
   helpOpen.set(next);
 };
 /** Subscribe to whether the sheet is showing (the Plex player suppresses wheel-volume under it). */
 export const useKeyboardHelpOpen = (): boolean => helpOpen.useValue();
-/** Whether the sheet was opened over a keyboard-owning layer (see `helpOverLayer`). */
-export const useKeyboardHelpOverLayer = (): boolean => helpOverLayer.useValue();
+/** What the sheet was opened over (see `helpOverLayer`). */
+export const useKeyboardHelpOverLayer = (): HelpOver => helpOverLayer.useValue();
 
 // The modules this account can reach — published by `useGlobalKeymap`, read by the `g` chords and
 // by the sheet (which drops the chord of a gated module rather than promise a page that isn't there).