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/playerModalHooks.ts

@@ -339,6 +339,7 @@ function playerTimes(p: YTPlayer | null): { cur: number; dur: number } | null {
 export function useYtProgress({
   activeId,
   resumeAt,
+  exactStart,
   mountRef,
   playerRef,
   currentIdRef,
@@ -355,6 +356,8 @@ export function useYtProgress({
 }: {
   activeId: string;
   resumeAt: number;
+  // `resumeAt` is where the user asked to start, so it beats a position this session stored.
+  exactStart: boolean;
   mountRef: RefObject<HTMLDivElement | null>;
   playerRef: RefObject<YTPlayer | null>;
   currentIdRef: RefObject<string>;
@@ -378,6 +381,7 @@ export function useYtProgress({
   // the end of the effect).
   const itemSnapshot = useEffectEvent(() => ({
     resumeAt,
+    exactStart,
     setWatched,
     setPlayerError,
     setLiveData,
@@ -392,6 +396,7 @@ export function useYtProgress({
     const id = activeId;
     const {
       resumeAt,
+      exactStart,
       setWatched,
       setPlayerError,
       setLiveData,
@@ -401,7 +406,8 @@ export function useYtProgress({
       qc,
     } = itemSnapshot();
     // A position this session already stored for the item beats the queue's snapshot of it.
-    const startAt = sessionPosRef.current.get(id) ?? resumeAt;
+    // …unless the user asked for a start of their own (Restart on a related row).
+    const startAt = exactStart ? resumeAt : (sessionPosRef.current.get(id) ?? resumeAt);
     autoMarkedRef.current = false;
     setPlayerError(null);