[dashboard] Unify Alert components & fix for dark theme

[jankeromnes]

Bug description

Currently, there are various implementations of the Alert component in the dashboard.

This causes them to appear non-uniform, especially in dark theme:

Steps to reproduce

1. Use dashboard in dark theme

Expected behavior

No response

Example repository

No response

[jankeromnes]

(Requires custom Werft preview with payments enabled, which makes testing this a bit tricky, so removing good-first-issue again.)