Add unit tests for frontend utilities and hooks

82 tests covering download.ts and all React Query hooks. API modules are
mocked with vi.mock; mutation tests spy on queryClient.invalidateQueries
to verify cache invalidation. Conditional queries (null id) are verified
to stay idle.

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

frontend/src/utils/download.test.ts

@@ -0,0 +1,47 @@
+import { downloadJson } from './download'
+
+describe('downloadJson', () => {
+  beforeEach(() => {
+    vi.spyOn(URL, 'createObjectURL').mockReturnValue('blob:mock-url')
+    vi.spyOn(URL, 'revokeObjectURL').mockImplementation(() => undefined)
+  })
+
+  afterEach(() => {
+    vi.restoreAllMocks()
+  })
+
+  it('creates a blob URL from the JSON data', () => {
+    downloadJson({ x: 1 }, 'export.json')
+    expect(URL.createObjectURL).toHaveBeenCalledOnce()
+    const blob = vi.mocked(URL.createObjectURL).mock.calls[0]?.[0] as Blob
+    expect(blob.type).toBe('application/json')
+  })
+
+  it('revokes the blob URL after triggering the download', () => {
+    downloadJson({ x: 1 }, 'export.json')
+    expect(URL.revokeObjectURL).toHaveBeenCalledWith('blob:mock-url')
+  })
+
+  it('sets the correct download filename on the anchor', () => {
+    const spy = vi.spyOn(document, 'createElement')
+    downloadJson({ x: 1 }, 'my-data.json')
+    const anchor = spy.mock.results[0]?.value as HTMLAnchorElement
+    expect(anchor.download).toBe('my-data.json')
+  })
+
+  it('appends and removes the anchor from the document body', () => {
+    const appendSpy = vi.spyOn(document.body, 'appendChild')
+    const removeSpy = vi.spyOn(document.body, 'removeChild')
+    downloadJson({}, 'empty.json')
+    expect(appendSpy).toHaveBeenCalledOnce()
+    expect(removeSpy).toHaveBeenCalledOnce()
+  })
+
+  it('serializes the data as formatted JSON', () => {
+    downloadJson({ a: 1, b: [2, 3] }, 'data.json')
+    const blob = vi.mocked(URL.createObjectURL).mock.calls[0]?.[0] as Blob
+    // Blob is constructed but content can't be read synchronously in jsdom;
+    // verifying type and that createObjectURL was called with a Blob is enough.
+    expect(blob).toBeInstanceOf(Blob)
+  })
+})