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/components/ui/Skeleton.tsx

@@ -13,6 +13,7 @@ function Skeleton({ className, inline = false }: { className?: string; inline?:
   return (
     <Tag
       aria-hidden="true"
+      data-skeleton=""
       className={clsx(
         "rounded-lg bg-fg/8 animate-pulse motion-reduce:animate-none",
         inline && "inline-block align-middle",