feat(frontend): display token usage per conversation turn (#1229)

Surface the usage_metadata that PR #1218 added to the streaming API.
A compact indicator in the chat header shows cumulative tokens consumed
per thread, with a tooltip breakdown of input/output/total counts.

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/i18n/locales/zh-CN.ts

@@ -261,6 +261,12 @@ export const zhCN: Translations = {
     failed: "子任务失败",
   },
 
+  // Token Usage
+  tokenUsage: {
+    title: "Token 用量",
+    input: "输入",
+    output: "输出",
+    total: "总计",
   // Shortcuts
   shortcuts: {
     searchActions: "搜索操作...",