v0.25.1 — button states, visible failures, no ghost persons

• A failed assignment no longer leaves an empty person behind. Assigning faces created
  the person first and assigned afterwards, so when nothing could be assigned — stale ids, an
  empty selection, or a second tab that got there first — the answer was HTTP 200 with
  {"assigned": 0} and the person stayed. Through "Track as new" that meant an empty
  Unnamed-xxxxxx in the gallery on every failed attempt, while the UI reported
  0 face(s) → Unnamed-ab12cd and looked like success. It now answers 409 and removes a
  person that was created for that very call; an existing person is never deleted, including
  when its display name or a different casing was typed.
• The action buttons show what they are doing. ASSIGN, ASSIGN ALL and "Track as new" had
  no hover, pressed or keyboard-focus style at all. Measured: 1.66:1 from rest to hover,
  1.50:1 from hover to pressed, with the label staying above 4.5:1 on both, and a focus ring
  at 13:1 against the panel. Pressing also shifts by 1px, which prefers-reduced-motion
  turns off.
• Failures are visible. A click whose request failed used to do nothing and say nothing:
  the request helper is awaited in 25 handlers, most without their own error handling. A
  single listener now reports every one of them, and still logs the original error to the
  console so a programming mistake keeps its stack trace.