Convert all store files from JavaScript to TypeScript

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/stores/files.test.js

@@ -28,7 +28,7 @@ vi.mock('@/api/client.js', () => ({
   default: { get: mockGet, post: mockPost, put: mockPut, delete: mockDelete },
 }));
 
-import { useFilesStore } from '@/stores/files.js';
+import { useFilesStore } from '@/stores/files';
 
 describe('filesStore', () => {
   let store;