fix(ui): a looping pulse stops for reduced motion; the e2e waits for skeleton bones, not for any pulse

- Seven looping pulses (a running job's dot, the scheduler's progress bar, a queued download, the
  download tray, a playlist upload, a channel's full-history action, the setup wizard's test mail)
  ran regardless of prefers-reduced-motion, which docs/DESIGN.md promises to respect: each is
  motion-safe now, held by a contract test.
- The a11y suite's "settled" wait took every aria-hidden pulse for a loading skeleton, so the
  Scheduler legend's running dot, which pulses for good, kept it waiting until it timed out. A
  skeleton bone now says it is one (data-skeleton) and the wait looks for that.

frontend/src/lib/reducedMotion.contract.test.ts

@@ -0,0 +1,34 @@
+import { readdirSync, readFileSync } from "node:fs";
+
+import { expect, it } from "vitest";
+
+// docs/DESIGN.md (Motion): `prefers-reduced-motion` is respected. A pulse that loops for as long as
+// something lasts — a running job's dot, a queued download, a skeleton bone — is the motion that rule
+// is about, and seven of them pulsed regardless. Each is `motion-safe:animate-pulse`, or pairs the
+// pulse with `motion-reduce:animate-none` on the same line.
+function sources(): Array<[string, string]> {
+  const walk = (u: URL, prefix: string): Array<[string, string]> =>
+    readdirSync(u, { withFileTypes: true }).flatMap((e) => {
+      if (e.isDirectory()) return walk(new URL(`${e.name}/`, u), `${prefix}${e.name}/`);
+      if (!/\.tsx?$/.test(e.name) || e.name.includes(".test.")) return [];
+      return [
+        [`${prefix}${e.name}`, readFileSync(new URL(e.name, u), "utf-8")] as [string, string],
+      ];
+    });
+  return walk(new URL("../", import.meta.url), "");
+}
+
+it("every looping pulse stops for a reader who asked for less motion", () => {
+  const unguarded = sources().flatMap(([name, src]) =>
+    src
+      .split("\n")
+      .map((line, i) => [line, i + 1] as const)
+      .filter(
+        ([line]) =>
+          /(?<!motion-safe:)\banimate-pulse\b/.test(line) &&
+          !line.includes("motion-reduce:animate-none"),
+      )
+      .map(([, n]) => `${name}:${n}`),
+  );
+  expect(unguarded).toEqual([]);
+});