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- Remove outdated Gemini model passthrough info cards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

frontend/src/types/index.ts

@@ -581,6 +581,7 @@ export interface GeminiCredentials {
   token_type?: string
   scope?: string
   expires_at?: string
+  model_mapping?: Record<string, string>
 }
 
 export interface TempUnschedulableRule {