feat: implement caching for Frankfurter API exchange rates

- Added a caching strategy for the Frankfurter API using Service Worker with StaleWhileRevalidate.
- Removed the local storage caching logic from the currency tool to streamline data fetching.
- Updated the currency tool to rely on the Service Worker for managing exchange rate data.

vite.config.ts

@@ -54,6 +54,21 @@ export default defineConfig(({ mode }) => ({
               }
             }
           },
+          {
+            // Frankfurter API 汇率缓存
+            urlPattern: /^https:\/\/api\.frankfurter\.app\/.*/i,
+            handler: 'StaleWhileRevalidate',
+            options: {
+              cacheName: 'currency-rates-cache',
+              expiration: {
+                maxEntries: 5,
+                maxAgeSeconds: 60 * 60 * 12 // 12 小时
+              },
+              cacheableResponse: {
+                statuses: [0, 200]
+              }
+            }
+          },
           {
             // Google Fonts 样式表缓存
             urlPattern: /^https:\/\/fonts\.googleapis\.com\/.*/i,