feat(ui): display three-level model mapping chain in usage logs

- Show channel + account mapping steps using model_mapping_chain field
- Add model_mapping_chain to AdminUsageLog TypeScript type
- Fallback to two-level display when chain is not available
- Fix cost nil guard in Anthropic/Antigravity RecordUsage paths
- Bump version to 0.1.105.31

frontend/src/types/index.ts

@@ -1054,6 +1054,7 @@ export interface UsageLogAccountSummary {
 
 export interface AdminUsageLog extends UsageLog {
   upstream_model?: string | null
+  model_mapping_chain?: string | null
 
   // 账号计费倍率（仅管理员可见）
   account_rate_multiplier?: number | null