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/tests/unit/core/threads/api.test.ts

@@ -0,0 +1,51 @@
+import { beforeEach, expect, test, vi } from "vitest";
+
+const fetchWithAuth = vi.fn();
+
+vi.mock("@/core/api/fetcher", () => ({
+  fetch: fetchWithAuth,
+}));
+
+beforeEach(() => {
+  fetchWithAuth.mockReset();
+});
+
+test("fetchThreadTokenUsage uses shared auth fetch without JSON GET headers", async () => {
+  fetchWithAuth.mockResolvedValue({
+    ok: true,
+    json: async () => ({
+      thread_id: "thread-1",
+      total_input_tokens: 3,
+      total_output_tokens: 4,
+      total_tokens: 7,
+      total_runs: 1,
+      by_model: { unknown: { tokens: 7, runs: 1 } },
+      by_caller: {},
+    }),
+  });
+
+  const { fetchThreadTokenUsage } = await import("@/core/threads/api");
+
+  await expect(fetchThreadTokenUsage("thread-1")).resolves.toMatchObject({
+    thread_id: "thread-1",
+    total_tokens: 7,
+  });
+
+  expect(fetchWithAuth).toHaveBeenCalledWith(
+    expect.stringContaining("/api/threads/thread-1/token-usage"),
+    {
+      method: "GET",
+    },
+  );
+});
+
+test("fetchThreadTokenUsage returns null for unavailable token usage", async () => {
+  fetchWithAuth.mockResolvedValue({
+    ok: false,
+    status: 404,
+  });
+
+  const { fetchThreadTokenUsage } = await import("@/core/threads/api");
+
+  await expect(fetchThreadTokenUsage("thread-1")).resolves.toBeNull();
+});