fix(playlists): "+N without length" counts every video the sum left out

The label said "without length" but the count held only live videos, so
a video not fetched yet, or gone before it was, was left out of the sum
and named nowhere. The server's `Video.without_length` and the client's
`isWithoutLength` are now one rule (no length, whatever the live
status), and the field, the key and the helpers are named for what they
count. The comments that still quoted "+N live" say the current copy.

frontend/src/features/video/withoutLength.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vitest";
+import type { TFunction } from "i18next";
+import { isWithoutLength, withLeftOutCount } from "./withoutLength";
+
+const t = ((key: string, o: { count: number }) => `${key}:${o.count}`) as unknown as TFunction;
+
+describe("a total and the videos it cannot hold", () => {
+  it("counts every video the server sent no length for", () => {
+    expect(isWithoutLength({ duration_seconds: null })).toBe(true);
+    expect(isWithoutLength({ duration_seconds: 5400 })).toBe(false);
+    expect(isWithoutLength({ duration_seconds: 0 })).toBe(false);
+  });
+
+  it("names them beside the sum, or alone when nothing summed", () => {
+    expect(withLeftOutCount("12 hr 5 min", 2, t)).toBe("12 hr 5 min card.withoutLength:2");
+    expect(withLeftOutCount("", 1, t)).toBe("card.withoutLength:1");
+    expect(withLeftOutCount("12 hr 5 min", 0, t)).toBe("12 hr 5 min");
+  });
+});