feat(ipsec): add certificate export functionality by name

- Implemented a new backend route for exporting existing client certificates in .p12 format by name.
- Enhanced the frontend to support exporting certificates directly from the IPsec server grid.
- Updated the IPsec page to manage certificate states and handle exports effectively.
- Introduced new utility functions for certificate handling and improved data structures to accommodate the changes.
- Added tests to ensure the reliability of the new certificate export feature.

components/ipsec/ipsec-user-sheet.tsx

@@ -151,7 +151,13 @@ function IpsecUserSheet({
               />
             </FormField>
             {!editing ? (
-              <FormField label="Аутентификация" required>
+              <FormField
+                label="Аутентификация"
+                required
+                hint={form.authMethod === "certificate"
+                  ? "CA и серверный серт берутся из уже настроенного IKEv2 (напр. MyCA + vpn-server); новый клиент подписывается этим CA"
+                  : undefined}
+              >
                 <div className="flex gap-1.5">
                   <Button
                     type="button"