test: add certificate feature unit tests and null-safety fix

Add comprehensive unit tests for the certificate upload, export,
and detail management feature:

- CertificateExportDialog: 21 tests covering format selection,
  blob download, error handling, and password-protected exports
- CertificateUploadDialog: 23 tests covering file validation,
  format detection, drag-and-drop, and upload flow
- CertificateDetailDialog: 19 tests covering detail display,
  loading state, missing fields, and branch coverage
- CertificateChainViewer: 8 tests covering chain visualization
- CertificateValidationPreview: 16 tests covering validation display
- FileDropZone: 18 tests covering drag-and-drop interactions
- useCertificates hooks: 10 tests covering all React Query hooks
- certificates API: 7 new tests for previously uncovered endpoints

Fix null-safety issue in ProxyHosts where cert.domains could be
undefined, causing a runtime error on split().

Frontend patch coverage: 90.6%, overall lines: 89.09%

frontend/src/components/ui/__tests__/FileDropZone.test.tsx

@@ -0,0 +1,157 @@
+import { render, screen, fireEvent } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+import { FileDropZone } from '../FileDropZone'
+
+vi.mock('react-i18next', () => ({
+  useTranslation: () => ({
+    t: (key: string) => key,
+    i18n: { language: 'en', changeLanguage: vi.fn() },
+  }),
+}))
+
+const defaultProps = {
+  id: 'cert-file',
+  label: 'Certificate File',
+  file: null as File | null,
+  onFileChange: vi.fn(),
+}
+
+function createFile(name = 'test.pem', type = 'application/x-pem-file'): File {
+  return new File(['cert-content'], name, { type })
+}
+
+describe('FileDropZone', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+  })
+
+  it('renders label and empty drop zone', () => {
+    render(<FileDropZone {...defaultProps} />)
+    expect(screen.getByText('Certificate File')).toBeTruthy()
+    expect(screen.getByText('certificates.dropFileHere')).toBeTruthy()
+  })
+
+  it('shows required asterisk when required', () => {
+    render(<FileDropZone {...defaultProps} required />)
+    expect(screen.getByText('*')).toBeTruthy()
+  })
+
+  it('displays file name when a file is provided', () => {
+    const file = createFile('my-cert.pem')
+    render(<FileDropZone {...defaultProps} file={file} />)
+    expect(screen.getByText('my-cert.pem')).toBeTruthy()
+  })
+
+  it('displays format badge when file is provided', () => {
+    const file = createFile('my-cert.pem')
+    render(<FileDropZone {...defaultProps} file={file} formatBadge="PEM" />)
+    expect(screen.getByText('PEM')).toBeTruthy()
+  })
+
+  it('triggers file input on click', async () => {
+    render(<FileDropZone {...defaultProps} />)
+    const dropZone = screen.getByRole('button')
+    await userEvent.click(dropZone)
+    // The hidden file input should exist
+    const input = document.getElementById('cert-file') as HTMLInputElement
+    expect(input).toBeTruthy()
+    expect(input.type).toBe('file')
+  })
+
+  it('calls onFileChange when a file is selected via input', () => {
+    render(<FileDropZone {...defaultProps} />)
+    const input = document.getElementById('cert-file') as HTMLInputElement
+    const file = createFile()
+    fireEvent.change(input, { target: { files: [file] } })
+    expect(defaultProps.onFileChange).toHaveBeenCalledWith(file)
+  })
+
+  it('calls onFileChange on drop', () => {
+    render(<FileDropZone {...defaultProps} />)
+    const dropZone = screen.getByRole('button')
+    const file = createFile()
+
+    fireEvent.dragOver(dropZone, { dataTransfer: { files: [file] } })
+    fireEvent.drop(dropZone, { dataTransfer: { files: [file] } })
+
+    expect(defaultProps.onFileChange).toHaveBeenCalledWith(file)
+  })
+
+  it('does not call onFileChange on drop when disabled', () => {
+    render(<FileDropZone {...defaultProps} disabled />)
+    const dropZone = screen.getByRole('button')
+    const file = createFile()
+
+    fireEvent.drop(dropZone, { dataTransfer: { files: [file] } })
+
+    expect(defaultProps.onFileChange).not.toHaveBeenCalled()
+  })
+
+  it('activates via keyboard Enter', async () => {
+    render(<FileDropZone {...defaultProps} />)
+    const dropZone = screen.getByRole('button')
+    fireEvent.keyDown(dropZone, { key: 'Enter' })
+    // Should not throw; input ref click would be called
+  })
+
+  it('activates via keyboard Space', async () => {
+    render(<FileDropZone {...defaultProps} />)
+    const dropZone = screen.getByRole('button')
+    fireEvent.keyDown(dropZone, { key: ' ' })
+  })
+
+  it('does not activate via keyboard when disabled', () => {
+    render(<FileDropZone {...defaultProps} disabled />)
+    const dropZone = screen.getByRole('button')
+    fireEvent.keyDown(dropZone, { key: 'Enter' })
+    // No crash, no file change
+    expect(defaultProps.onFileChange).not.toHaveBeenCalled()
+  })
+
+  it('sets aria-disabled when disabled', () => {
+    render(<FileDropZone {...defaultProps} disabled />)
+    const dropZone = screen.getByRole('button')
+    expect(dropZone.getAttribute('aria-disabled')).toBe('true')
+  })
+
+  it('has tabIndex=-1 when disabled', () => {
+    render(<FileDropZone {...defaultProps} disabled />)
+    const dropZone = screen.getByRole('button')
+    expect(dropZone.tabIndex).toBe(-1)
+  })
+
+  it('has tabIndex=0 when not disabled', () => {
+    render(<FileDropZone {...defaultProps} />)
+    const dropZone = screen.getByRole('button')
+    expect(dropZone.tabIndex).toBe(0)
+  })
+
+  it('has appropriate aria-label when file is selected', () => {
+    const file = createFile('cert.pem')
+    render(<FileDropZone {...defaultProps} file={file} />)
+    const dropZone = screen.getByRole('button')
+    expect(dropZone.getAttribute('aria-label')).toBe('Certificate File: cert.pem')
+  })
+
+  it('handles dragLeave event', () => {
+    render(<FileDropZone {...defaultProps} />)
+    const dropZone = screen.getByRole('button')
+    fireEvent.dragOver(dropZone, { dataTransfer: { files: [] } })
+    fireEvent.dragLeave(dropZone)
+    // No crash; drag state should reset
+  })
+
+  it('sets accept attribute on input', () => {
+    render(<FileDropZone {...defaultProps} accept=".pem,.crt" />)
+    const input = document.getElementById('cert-file') as HTMLInputElement
+    expect(input.getAttribute('accept')).toBe('.pem,.crt')
+  })
+
+  it('sets aria-required on input when required', () => {
+    render(<FileDropZone {...defaultProps} required />)
+    const input = document.getElementById('cert-file') as HTMLInputElement
+    expect(input.getAttribute('aria-required')).toBe('true')
+  })
+})