feat(dashboard): 最近请求缓存命中列 + 中文模式人民币计价 + 价格解析记忆化

- 最近请求表新增缓存命中率列（缓存读取/总输入）
- useCurrency：中文界面费用按最新 USD→CNY 汇率显示 ¥，英文保持 $（open.er-api.com，缓存 12h，失败兜底 7.2）
- match_price 记忆化缓存：O(记录数×5910) → O(模型种类×5910)，扫描从秒级降到百毫秒级
- 仪表盘右上角显示加载耗时/数据量（get_summary 往返埋点）；Rust 侧分段计时

src/hooks/useDashboard.ts

@@ -22,16 +22,23 @@ export function useDashboard() {
   const [data, setData] = useState<SummaryResult | null>(null);
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  /** Round-trip timing for the last get_summary call (user-perceived lag). */
+  const [loadStats, setLoadStats] = useState<{ ms: number; kb: number } | null>(null);
 
   const refresh = useCallback(async (force = false) => {
     setLoading(true);
     setError(null);
+    const start = performance.now();
     try {
       const result = await invoke<SummaryResult>("get_summary", {
         range,
         refresh: force,
       });
       setData(result);
+      setLoadStats({
+        ms: Math.round(performance.now() - start),
+        kb: Math.round(JSON.stringify(result).length / 1024),
+      });
     } catch (err) {
       const msg = err instanceof Error ? err.message : String(err);
       setError(msg);
@@ -53,5 +60,5 @@ export function useDashboard() {
     }
   }, []);
 
-  return { range, changeRange, data, loading, error, refresh };
+  return { range, changeRange, data, loading, error, refresh, loadStats };
 }