[Login] Insufficient contrast for the error banner

[g123k]

Hi everyone,

In the Login screen, the error label is poorly contrasted:

A contrast checker for this screen gives:

[monsieurtanuki]

"Incorrect username or password" is an error and should be considered as such, with standard error and onError colors (which is not the case currently).

• cf ColorScheme.light (which passes the AAA test) (I used https://webaim.org/resources/contrastchecker/)
  • error = const Color(0xffb00020)
  • onError = Colors.white
• cf ColorScheme.dark (which does not pass the AAA test)
  • error = const Color(0xffcf6679) // with red[300] = Color(0xFFE57373) the test would pass
  • onError = Colors.black