feat(frontend): display upstream model in usage table and distribution charts

Show upstream model mapping (requested -> upstream) in UsageTable with arrow notation. Add requested/upstream/mapping source toggle to ModelDistributionChart with lazy loading — only fetches data when user switches tab, with per-source cache invalidation on filter changes. Include upstream_model column in Excel export and i18n for en/zh.

frontend/src/types/index.ts

@@ -975,6 +975,7 @@ export interface UsageLog {
   account_id: number | null
   request_id: string
   model: string
+  upstream_model?: string | null
   service_tier?: string | null
   reasoning_effort?: string | null
   inbound_endpoint?: string | null