feat: french translation (#802)

* Add French (fr) language support

- Create app/src/i18n/locales/fr/translation.json with full UI translations
- Register French in SUPPORTED_LANGUAGES and i18next resources
- Wire French locale from date-fns for relative date formatting

* Fix Vite file watcher EBUSY error on Windows by excluding Rust target directory

app/src/lib/utils/format.ts

@@ -1,5 +1,5 @@
 import { formatDistance } from 'date-fns';
-import { ja, zhCN, zhTW } from 'date-fns/locale';
+import { ja, zhCN, zhTW, fr } from 'date-fns/locale';
 import i18n from '@/i18n';
 
 export function formatDuration(seconds: number): string {
@@ -16,6 +16,8 @@ function getDateLocale() {
       return zhCN;
     case 'zh-TW':
       return zhTW;
+    case 'fr':
+      return fr;
     default:
       return undefined;
   }