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.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { afterRemoval } from "./playerQueue";
+import { afterRemoval, stepIndex } from "./playerQueue";
 import type { Video } from "./api";
 
 // "Delete" from inside the player (the owner asked for it: today the only way to drop a video from a
@@ -51,3 +51,25 @@ describe("afterRemoval", () => {
     expect(next!.playingId).toBe("a");
   });
 });
+
+describe("stepIndex", () => {
+  const q = [v("a"), v("b"), v("c"), v("d")];
+  const none = new Set<string>();
+  it("steps to the neighbour that way", () => {
+    expect(stepIndex(q, 1, 1, none)).toBe(2);
+    expect(stepIndex(q, 1, -1, none)).toBe(0);
+  });
+  it("passes over a video known dead", () => {
+    expect(stepIndex(q, 3, -1, new Set(["c", "b"]))).toBe(0);
+    expect(stepIndex(q, 0, 1, new Set(["b"]))).toBe(2);
+  });
+  it("is -1 at the end, or when only dead videos are left that way", () => {
+    expect(stepIndex(q, 3, 1, none)).toBe(-1);
+    expect(stepIndex(q, 2, -1, new Set(["a", "b"]))).toBe(-1);
+  });
+  it("wraps when asked, and never lands on where it started", () => {
+    expect(stepIndex(q, 3, 1, none, true)).toBe(0);
+    expect(stepIndex(q, 0, -1, new Set(["d"]), true)).toBe(2);
+    expect(stepIndex(q, 1, 1, new Set(["a", "c", "d"]), true)).toBe(-1);
+  });
+});