feat(admin): surface reset credit expiry and redemption results

[rexdotsh]

What

Adds more context around Codex reset credits in the admin UI. Verified against OpenAI's Codex CLI source that we already decode every field the API exposes - this just surfaces what was being thrown away at display time.

• Sort available reset credits by soonest expiry (matches Codex TUI behavior) and show a reset credit expires in Xd chip in the limits footer
• Include the credit description and expiry in the restore button tooltip
• Decode redemption result codes into human-readable toasts:
  • reset -> "Limits restored (N windows reset)" (success)
  • no_credit -> error toast
  • nothing_to_reset / already_redeemed -> neutral toasts with window count when present

Notes

A reset is instantaneous (windows are zeroed on consume), so there is no "reset duration" to track - the credit's own expires_at is the only lifetime involved, and it is now visible.