Add frontend testing infrastructure with Vitest (#14)

frontend/src/test/setup.ts

@@ -0,0 +1,37 @@
+import '@testing-library/jest-dom';
+
+class MockDataTransfer implements DataTransfer {
+  dropEffect: DataTransfer['dropEffect'] = 'none';
+  effectAllowed: DataTransfer['effectAllowed'] = 'all';
+  files: FileList = Object.assign([], { item: (i: number) => this.files[i] || null });
+  items: DataTransferItemList = Object.assign([], {
+    add: () => null,
+    remove: () => {},
+    clear: () => {},
+    item: () => null,
+  }) as unknown as DataTransferItemList;
+  types: readonly string[] = [];
+
+  clearData(): void {}
+  getData(): string { return ''; }
+  setData(): void {}
+  setDragImage(): void {}
+}
+
+Object.defineProperty(global, 'DataTransfer', {
+  value: MockDataTransfer,
+});
+
+Object.defineProperty(window, 'matchMedia', {
+  writable: true,
+  value: (query: string) => ({
+    matches: false,
+    media: query,
+    onchange: null,
+    addListener: () => {},
+    removeListener: () => {},
+    addEventListener: () => {},
+    removeEventListener: () => {},
+    dispatchEvent: () => false,
+  }),
+});