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/components/workspace/token-usage-indicator.tsx

@@ -0,0 +1,74 @@
+"use client";
+
+import type { Message } from "@langchain/langgraph-sdk";
+import { CoinsIcon } from "lucide-react";
+import { useMemo } from "react";
+
+import {
+  Tooltip,
+  TooltipContent,
+  TooltipTrigger,
+} from "@/components/ui/tooltip";
+import { useI18n } from "@/core/i18n/hooks";
+import { accumulateUsage, formatTokenCount } from "@/core/messages/usage";
+import { cn } from "@/lib/utils";
+
+interface TokenUsageIndicatorProps {
+  messages: Message[];
+  className?: string;
+}
+
+export function TokenUsageIndicator({
+  messages,
+  className,
+}: TokenUsageIndicatorProps) {
+  const { t } = useI18n();
+
+  const usage = useMemo(() => accumulateUsage(messages), [messages]);
+
+  if (!usage) {
+    return null;
+  }
+
+  return (
+    <Tooltip delayDuration={200}>
+      <TooltipTrigger asChild>
+        <button
+          type="button"
+          className={cn(
+            "text-muted-foreground flex cursor-default items-center gap-1 text-xs",
+            className,
+          )}
+        >
+          <CoinsIcon size={14} />
+          <span>{formatTokenCount(usage.totalTokens)}</span>
+        </button>
+      </TooltipTrigger>
+      <TooltipContent side="bottom" align="end">
+        <div className="space-y-1 text-xs">
+          <div className="font-medium">{t.tokenUsage.title}</div>
+          <div className="flex justify-between gap-4">
+            <span>{t.tokenUsage.input}</span>
+            <span className="font-mono">
+              {formatTokenCount(usage.inputTokens)}
+            </span>
+          </div>
+          <div className="flex justify-between gap-4">
+            <span>{t.tokenUsage.output}</span>
+            <span className="font-mono">
+              {formatTokenCount(usage.outputTokens)}
+            </span>
+          </div>
+          <div className="border-t pt-1">
+            <div className="flex justify-between gap-4">
+              <span>{t.tokenUsage.total}</span>
+              <span className="font-mono font-medium">
+                {formatTokenCount(usage.totalTokens)}
+              </span>
+            </div>
+          </div>
+        </div>
+      </TooltipContent>
+    </Tooltip>
+  );
+}