feat: add certificate export and upload dialogs

- Implemented CertificateExportDialog for exporting certificates in various formats (PEM, PFX, DER) with options to include private keys and set passwords.
- Created CertificateUploadDialog for uploading certificates, including validation and support for multiple file types (certificates, private keys, chain files).
- Updated DeleteCertificateDialog to use 'domains' instead of 'domain' for consistency.
- Refactored BulkDeleteCertificateDialog and DeleteCertificateDialog tests to accommodate changes in certificate structure.
- Added FileDropZone component for improved file upload experience.
- Enhanced translation files with new keys for certificate management features.
- Updated Certificates page to utilize the new CertificateUploadDialog and clean up the upload logic.
- Adjusted Dashboard and ProxyHosts pages to reflect changes in certificate data structure.

frontend/src/pages/__tests__/ProxyHosts-coverage.test.tsx

@@ -485,8 +485,8 @@ describe('ProxyHosts - Coverage enhancements', () => {
 
     vi.mocked(proxyHostsApi.getProxyHosts).mockResolvedValue([hostCustom, hostStaging, hostAuto, hostLets])
     vi.mocked(certificatesApi.getCertificates).mockResolvedValue([
-      { domain: 'staging.com', status: 'untrusted', provider: 'letsencrypt-staging', issuer: 'Let\'s Encrypt', expires_at: '2026-01-01' },
-      { domain: 'lets.com', status: 'valid', provider: 'letsencrypt', issuer: 'Let\'s Encrypt', expires_at: '2026-01-01' },
+      { uuid: 'cert-staging', domains: 'staging.com', status: 'untrusted', provider: 'letsencrypt-staging', issuer: 'Let\'s Encrypt', expires_at: '2026-01-01', has_key: false, in_use: true },
+      { uuid: 'cert-lets', domains: 'lets.com', status: 'valid', provider: 'letsencrypt', issuer: 'Let\'s Encrypt', expires_at: '2026-01-01', has_key: false, in_use: true },
     ])
     vi.mocked(accessListsApi.accessListsApi.list).mockResolvedValue([])
     vi.mocked(settingsApi.getSettings).mockResolvedValue({})