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/utils/email.ts

@@ -334,8 +334,8 @@ let emailServiceInstance: EmailService | null = null;
 /**
  * Get or create EmailService instance with current SMTP config
  */
-export function getEmailService(): EmailService {
-  const { getSMTPConfig } = require('./admin-config');
+export async function getEmailService(): Promise<EmailService> {
+  const { getSMTPConfig } = await import('./admin-config');
   const config = getSMTPConfig();
   
   if (!emailServiceInstance) {