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/components/ProxyHostForm.tsx

@@ -917,8 +917,8 @@ export default function ProxyHostForm({ host, onSubmit, onCancel }: ProxyHostFor
               <SelectContent>
                 <SelectItem value="0">Auto-manage with Let's Encrypt (recommended)</SelectItem>
                 {certificates.map(cert => (
-                  <SelectItem key={cert.id || cert.domain} value={String(cert.id ?? 0)}>
-                    {(cert.name || cert.domain)}
+                  <SelectItem key={cert.id || cert.domains} value={String(cert.id ?? 0)}>
+                    {(cert.name || cert.domains)}
                     {cert.provider ? ` (${cert.provider})` : ''}
                   </SelectItem>
                 ))}