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/threads/token-usage.ts

@@ -0,0 +1,20 @@
+import type { TokenUsage } from "@/core/messages/usage";
+
+import type { ThreadTokenUsageResponse } from "./types";
+
+export function threadTokenUsageQueryKey(threadId?: string | null) {
+  return ["thread-token-usage", threadId] as const;
+}
+
+export function threadTokenUsageToTokenUsage(
+  usage: ThreadTokenUsageResponse | null | undefined,
+): TokenUsage | null {
+  if (!usage) {
+    return null;
+  }
+  return {
+    inputTokens: usage.total_input_tokens ?? 0,
+    outputTokens: usage.total_output_tokens ?? 0,
+    totalTokens: usage.total_tokens ?? 0,
+  };
+}