Add URL format hint and error feedback for table links

[silverkszlo]

When a user wants to add a link to a table, the format constraint requires the string to start with "https://...". The input field will not accept entries like "google.com". If a user tries to save an incorrectly formatted link, the edit field simply closes without showing an error message and, obviously, without saving the invalid input.

It would be helpful to provide a hint for the user, for example as a placeholder, to indicate the correct format. Additionally, if the user attempts to save a wrongly formatted input, an error message should be displayed.