feat: Add email settings management and templates functionality

- Implemented email settings configuration in the admin panel, allowing for SMTP settings and notification preferences.
- Created a new template for email settings with fields for SMTP host, port, username, password, sender address, and recipients.
- Added JavaScript functionality to handle loading, saving, and validating email settings.
- Introduced email templates management, enabling the listing, editing, and saving of email templates.
- Updated navigation to include links to email settings and templates.
- Added tests for email settings and templates to ensure proper functionality and validation.

.env.example

@@ -17,6 +17,9 @@ SMTP_PASSWORD=your-password
 SMTP_SENDER=web@example.com
 SMTP_RECIPIENTS=team@example.com
 SMTP_USE_TLS=true
+# Default notification toggles (admin UI can override these at runtime)
+EMAIL_NOTIFY_CONTACT_FORM=true
+EMAIL_NOTIFY_NEWSLETTER_SIGNUPS=false
 # Set to 1 to enable SMTP integration tests during CI/CD (requires valid SMTP settings)
 RUN_SMTP_INTEGRATION_TEST=0