auth: extract reusable sendVerificationEmail utility, enable sendOnSignUp

- Create src/lib/server/email-verification.ts with exported utility function
- Use it in auth.ts sendVerificationEmail callback
- Enable sendOnSignUp so verification email is sent on registration

src/lib/server/email-verification.ts

@@ -0,0 +1,15 @@
+import { sendMail } from './resend';
+
+export async function sendVerificationEmail(options: {
+	email: string;
+	verificationUrl: string;
+}) {
+	await sendMail({
+		to: options.email,
+		templateId: 'email-verification-email',
+		data: {
+			target_email_address: options.email,
+			verification_link: options.verificationUrl
+		}
+	});
+}