fix: include retry timing in 429 error messages (#366)

## Summary

- Extract retry-after header from 429 API errors and include timing
  guidance in the user-facing error message
- Previously, non-quota 429 errors showed a generic message with no
  guidance on when to retry, only a link to status.anthropic.com

## Impact

- user-facing impact: 429 error messages now tell users when to retry
  instead of just linking to a status page
- developer/maintainer impact: none

## Testing

- [x] `bun run build`
- [ ] `bun run smoke`
- [ ] focused tests: error formatting is pure string construction,
  verified via build + manual inspection

## Notes

- provider/model path tested: applies to all providers returning 429
- screenshots attached (if UI changed): n/a
- follow-up work or known limitations: 529 errors could get similar
  treatment in a follow-up

https://claude.ai/code/session_01D7kprMn4c66a5WrZscF7rv

Co-authored-by: Claude <noreply@anthropic.com>

src/services/api/errors.ts

@@ -557,8 +557,12 @@ export function getAssistantMessageFromError(
     const stripped = error.message.replace(/^429\s+/, '')
     const innerMessage = stripped.match(/"message"\s*:\s*"([^"]*)"/)?.[1]
     const detail = innerMessage || stripped
+    const retryAfter = (error as APIError).headers?.get?.('retry-after')
+    const retryHint = retryAfter && !isNaN(Number(retryAfter))
+      ? `Try again in ${retryAfter} seconds.`
+      : 'Try again in a few seconds.'
     return createAssistantAPIErrorMessage({
-      content: `${API_ERROR_MESSAGE_PREFIX}: Request rejected (429) · ${detail || `this may be a temporary capacity issue${getAPIProvider() === 'firstParty' ? ' — check status.anthropic.com' : ''}`}`,
+      content: `${API_ERROR_MESSAGE_PREFIX}: Request rejected (429) · ${detail || 'this may be a temporary capacity issue'} — ${retryHint}`,
       error: 'rate_limit',
     })
   }