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

backend/app/models/entities.py

@@ -136,7 +136,7 @@ class MailSetting(Base):
     smtp_port: Mapped[int] = mapped_column(Integer, default=587)
     smtp_user: Mapped[str | None] = mapped_column(String(220), nullable=True)
     smtp_password_encrypted: Mapped[str | None] = mapped_column(Text, nullable=True)
-    use_tls: Mapped[bool] = mapped_column(Boolean, default=True)
+    use_tls: Mapped[bool] = mapped_column(Boolean, default=False)
     use_starttls: Mapped[bool] = mapped_column(Boolean, default=True)
     sender_address: Mapped[str | None] = mapped_column(String(320), nullable=True)
     sender_name: Mapped[str] = mapped_column(String(160), default="NexaPantry")