fix: enable Gemini model_mapping UI and extend warmup to Antigravity

- Remove Gemini platform exclusion from model restriction UI in
  Create/Edit account modals (Gemini now supports model_mapping)
- Remove outdated Gemini model passthrough info cards
- Add model_mapping field to GeminiCredentials type
- Extend warmup request interception toggle to Antigravity platform
- Remove redundant try/catch in API key account creation
- Remove noisy gateway.request_completed debug log
- Reorganize Gemini model mapping sections in constants.go

backend/internal/handler/gateway_handler.go

@@ -650,11 +650,6 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 					).Error("gateway.record_usage_failed", zap.Error(err))
 				}
 			})
-			reqLog.Debug("gateway.request_completed",
-				zap.Int64("account_id", account.ID),
-				zap.Int("switch_count", fs.SwitchCount),
-				zap.Bool("fallback_used", fallbackUsed),
-			)
 			return
 		}
 		if !retryWithFallback {