fix: improve SMTP configuration and error handling

- Change default use_tls from True to False to match typical STARTTLS setup
- Add MailDeliveryError exception for mail delivery failures
- Wrap send_mail calls in try-catch blocks to handle errors gracefully
- Return 502 status code with error details when mail delivery fails
- Add SMTP security mode selector in frontend (STARTTLS/TLS/None)
- Add test mail form to admin panel
- Handle empty SMTP credentials properly in update_mail_settings
- Catch

frontend/src/i18n/dictionaries.ts

@@ -61,6 +61,10 @@ export const dictionaries: Record<Language, Record<string, string>> = {
     smtpPort: 'SMTP Port',
     smtpUser: 'SMTP User',
     smtpPassword: 'SMTP Passwort',
+    smtpSecurity: 'SMTP Sicherheit',
+    smtpStarttls: 'STARTTLS (typisch Port 587)',
+    smtpTls: 'TLS/SSL (typisch Port 465)',
+    smtpNone: 'Keine Verschlüsselung',
     senderAddress: 'Absender-Adresse',
     senderName: 'Absender-Name',
     testMail: 'Testmail senden',
@@ -128,6 +132,10 @@ export const dictionaries: Record<Language, Record<string, string>> = {
     smtpPort: 'SMTP port',
     smtpUser: 'SMTP user',
     smtpPassword: 'SMTP password',
+    smtpSecurity: 'SMTP security',
+    smtpStarttls: 'STARTTLS (usually port 587)',
+    smtpTls: 'TLS/SSL (usually port 465)',
+    smtpNone: 'No encryption',
     senderAddress: 'Sender address',
     senderName: 'Sender name',
     testMail: 'Send test mail',