fix(player): related marks survive, Restart restarts, and refused pages offer nothing to click

- Every state and Watch-later write goes through features/video/videoWrites,
  which marks the never-refetched lists (related lists, live searches) once
  the server has it; a contract test holds it. The Notification Center's
  undo, the feed's reset and the Watch-later toggles skipped it before.
- Confirmed marks are also kept apart (qk.videoMarks) and laid over the
  related rows: a Show more page landing later rebuilt them from the pages
  it read when it started, and dropped a mark made meanwhile.
- Restart on a related video starts at 0 (the session's stored position
  won before), and on the one already playing it seeks.
- The watched toggle remembers each video's mark: Back showed the opened
  video's launch-time state after a related one was marked.
- The help sheet leaves out the player's keys while a layer above it has
  them waiting.
- After a refusal a retry cannot answer, neither Show more (related) nor
  Load more (live search) is offered; a failed live-search page keeps the
  results already read and says why under them.
- The related cap message counts requests, as the cap does.

frontend/src/lib/keymap.ts

@@ -488,17 +488,31 @@ export const setListInTopLayerProbe = (probe: () => boolean): void => {
   listInTopLayer = probe;
 };
 
+// A surface whose keys wait while a layer sits above it (the YouTube player under its related
+// drawer or a dialog: every key but Escape is the layer's then). Its rows would promise keys that do
+// nothing, so the sheet leaves the scope out — asked at the same moment as `helpOverLayer`.
+const coveredProbes = new Map<KeyScope, () => boolean>();
+export const setScopeCoveredProbe = (scope: KeyScope, probe: (() => boolean) | null): void => {
+  if (probe) coveredProbes.set(scope, probe);
+  else coveredProbes.delete(scope);
+};
+const helpCovered = createStore<ReadonlySet<KeyScope>>(new Set());
+
 export const closeKeyboardHelp = (): void => helpOpen.set(false);
 export const toggleKeyboardHelp = (): void => {
   const next = !helpOpen.get();
-  if (next)
+  if (next) {
     helpOverLayer.set(!anyBlockingLayer() ? "page" : listInTopLayer() ? "layerWithList" : "layer");
+    helpCovered.set(new Set([...coveredProbes].filter(([, covered]) => covered()).map(([s]) => s)));
+  }
   helpOpen.set(next);
 };
 /** Subscribe to whether the sheet is showing (the Plex player suppresses wheel-volume under it). */
 export const useKeyboardHelpOpen = (): boolean => helpOpen.useValue();
 /** What the sheet was opened over (see `helpOverLayer`). */
 export const useKeyboardHelpOverLayer = (): HelpOver => helpOverLayer.useValue();
+/** The surfaces a layer covered when the sheet opened (see `setScopeCoveredProbe`). */
+export const useKeyboardHelpCovered = (): ReadonlySet<KeyScope> => helpCovered.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).