fix: use backend thread token usage for header total (#2800)

* fix: use backend thread token usage for header total

* Refactor thread token usage fetch

frontend/src/core/i18n/locales/en-US.ts

@@ -310,7 +310,7 @@ export const enUS: Translations = {
     unavailable:
       "No token usage yet. Usage appears only after a successful model response when the provider returns usage_metadata.",
     unavailableShort: "No usage returned",
-    note: "Shown from provider-returned usage_metadata. Totals are best-effort conversation totals and may differ from provider billing pages.",
+    note: "Header totals use persisted thread usage when available. Per-turn and debug usage come from visible messages. Totals may differ from provider billing pages.",
     presets: {
       off: "Off",
       summary: "Summary",

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

@@ -296,7 +296,7 @@ export const zhCN: Translations = {
     unavailable:
       "暂无 Token 用量。只有模型成功返回且供应商提供 usage_metadata 时才会显示。",
     unavailableShort: "未返回用量",
-    note: "基于供应商返回的 usage_metadata 展示。当前总量是 best-effort 的会话参考值，可能与平台账单页不完全一致。",
+    note: "顶部总量优先使用后端持久化的线程用量。每轮和调试用量来自当前可见消息，可能与平台账单页不完全一致。",
     presets: {
       off: "关闭",
       summary: "总览",