fix : remove double /api prefix in export URL

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/services/time-entries.ts

@@ -71,7 +71,7 @@ export function useTimeEntryService() {
         if (params.tags?.length) {
             params.tags.forEach(id => query.append('tags[]', String(id)))
         }
-        return `/api/time_entries/export?${query.toString()}`
+        return `/time_entries/export?${query.toString()}`
     }
 
     async function downloadExport(params: {