Make email service initialization asynchronous

Convert getEmailService() to async function and update all callers to use await.
Replace synchronous require() with dynamic import() for admin-config module.
Add SMTP config loading to admin configuration dialog.

server/api/admin/test-email.post.ts

@@ -49,7 +49,7 @@ export default defineEventHandler(async (event) => {
 
     // Get email service and send test email
     const { getEmailService } = await import('~/server/utils/email');
-    const emailService = getEmailService();
+    const emailService = await getEmailService();
     
     // Verify connection first
     const connectionOk = await emailService.verifyConnection();