fix(player): a step onto an unplayable video carries on the way it went

Previous onto an embed-blocked or removed video skipped forward again (the
auto-advance direction), straight back to the item just left. The player now
remembers the direction of the step that reached the item and the videos
YouTube refused this session: an error carries on that way, Previous / Next
pass over known-dead items without loading them, and go dark when only those
are left. A step of your own skips with auto-advance off too.

frontend/src/lib/playerQueue.ts

@@ -28,3 +28,25 @@ export function afterRemoval(
   const next = rest[Math.min(Math.max(index, 0), rest.length - 1)]!;
   return { queue: rest, playingId: next.id };
 }
+
+/** The nearest item from `index` going `dir` that is not known to be unplayable, or -1 when there is
+ *  none that way. `wrap` goes round the ends (Loop "All"), never back onto `index` itself.
+ *
+ * A video YouTube refused to play (embed-blocked, removed) is remembered for the player session, so
+ * Previous / Next pass over it instead of loading it again only to be bounced off it. */
+export function stepIndex(
+  queue: readonly Video[],
+  index: number,
+  dir: 1 | -1,
+  dead: ReadonlySet<string>,
+  wrap = false,
+): number {
+  const n = queue.length;
+  for (let k = 1; k < n; k++) {
+    const raw = index + dir * k;
+    if (!wrap && (raw < 0 || raw >= n)) return -1;
+    const j = ((raw % n) + n) % n;
+    if (!dead.has(queue[j]!.id)) return j;
+  }
+  return -1;
+}