1. Frontend: replace hardcoded antigravityDefaultMappings with async
fetch from GET /admin/accounts/antigravity/default-model-mapping,
eliminating the duplicate data source that caused frontend/backend
mapping inconsistency.
2. Backend: convert handleSmartRetry and antigravityRetryLoop from
standalone functions to AntigravityGatewayService methods, enabling
Redis cache sync (updateAccountModelRateLimitInCache) after both
rate-limit write paths — long-delay branch and retry-exhausted branch.