Border for account signature editor

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a Nextcloud Mail user I want to set and later change my signature. The current design makes it tricky to see where the editor starts and ends. E.g. Place signature above quoted text can look like it's part of the signature.

Describe the solution you'd like

Wrap the editor with the default text input border

Describe alternatives you've considered

No response

Additional context

No response