Let us configure how fast spinner verbs rotate

What I want

A setting to control how quickly the spinner verbs cycle. Right now I can pick which verbs show up via spinnerVerbs, but they rotate at a fixed speed I can't change.

Why

I have a small custom verb list and the current rotation feels sluggish. I'd like them to cycle faster. Other people with longer lists might want the opposite — slower rotation so they can actually read each one.

Suggestion

A top-level spinnerVerbInterval in settings.json, value in milliseconds:

{
  "spinnerVerbs": {
    "mode": "replace",
    "verbs": ["Thinking", "Processing", "Computing"]
  },
  "spinnerVerbInterval": 1000
}

Should probably have a sane minimum (200ms or so) to avoid flickering. Default to whatever it is today so nothing changes for existing users.

Other options I thought about

Could also nest it inside the spinnerVerbs object as "interval", but a top-level key is simpler and doesn't change the existing schema shape.

Per-verb durations would be cool but way overkill.

Context

The current spinner customization surface is spinnerVerbs, spinnerTipsEnabled, and prefersReducedMotion. This would be a natural addition alongside those.