Extra usage widgets hardcode $ instead of using the API's currency field #415

Summary

The usage API reports the account's billing currency ("currency": "EUR" in my case), but ExtraUsageRemaining formats with a hardcoded $. Once a monthly limit is set, an EUR account sees its euros displayed with a dollar sign.

Raw API response (/api/oauth/usage, trimmed)

"extra_usage": {
  "is_enabled": true,
  "monthly_limit": null,
  "used_credits": 542.0,
  "utilization": null,
  "currency": "EUR",
  "disabled_reason": null
}

Suggested fix

Parse the currency field (it isn't currently in UsageApiResponseSchema/CachedUsageDataSchema) and format with the matching symbol, falling back to $/the raw currency code when unknown.

Related to but distinct from #387, which fixed the cents/dollars math. Also related: #413, #414.