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/api/core.ts

@@ -65,6 +65,12 @@ export class HttpError extends Error {
   }
 }
 
+/** The structured refusal's reason (`{ code, reason }`), when the server gave one. */
+export const refusalReason = (err: unknown): string | undefined =>
+  err instanceof HttpError
+    ? (err.detailData as { reason?: string } | undefined)?.reason
+    : undefined;
+
 // Localize a STRUCTURED server error ({ code, reason, ... }) into the user's language. FastAPI
 // usually returns a plain string `detail`, but some routes (e.g. download quota/edit) return a
 // structured object with an i18n reason key + numbers so HU/DE users don't get hardcoded English