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/schemas/common.py

@@ -139,7 +139,7 @@ class MailSettingsIn(BaseModel):
     smtp_port: int = Field(default=587, ge=1, le=65535)
     smtp_user: str | None = Field(default=None, max_length=220)
     smtp_password: str | None = Field(default=None, max_length=1000)
-    use_tls: bool = True
+    use_tls: bool = False
     use_starttls: bool = True
     sender_address: EmailStr | None = None
     sender_name: str = Field(default="NexaPantry", max_length=160)