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/i18n/locales/zh.ts

@@ -723,11 +723,14 @@ export default {
     exporting: '导出中...',
     preparingExport: '正在准备导出...',
     model: '模型',
+    requestedModel: '请求',
+    upstreamModel: '上游',
     reasoningEffort: '推理强度',
     endpoint: '端点',
     endpointDistribution: '端点分布',
     inbound: '入站',
     upstream: '上游',
+    mapping: '映射',
     path: '路径',
     inboundEndpoint: '入站端点',
     upstreamEndpoint: '上游端点',