Merge pull request #21 from appalexhu/feat/proactive-auto-switch

feat: Proactive auto-switch before hitting rate limits (opt-in)

Conflict in AppState.swift resolved by keeping both blocks of state:
#22's usage-polling state (isRefreshing / usageFetchCursor /
usageRetryNotBefore) and #21's auto-switch state. Purely additive,
no behaviour from either PR was changed in the resolution.

Two known interactions with #22 are NOT fixed here and are handled in
a follow-up commit, so that each contributor's work stays intact:

1. evaluateAutoSwitch() runs at the end of refresh(), while #22's
   isRefreshing guard is still true, so the switchTo() -> refresh()
   that should update state after an automatic switch is skipped.

2. #21 treats a candidate with no usage sample as eligible. That was
   safe when every account was polled every cycle; with #22's
   round-robin a candidate can be unsampled or minutes stale, so an
   automatic switch can land on an account that is itself maxed out.

Auto-switch is opt-in and defaults to off, so the default path is
unaffected either way.