A11y: dark mode contrast, sign-in landmark/h1, Banner link discernibility

lib/mv_web/auth_overrides.ex

@@ -32,10 +32,11 @@ defmodule MvWeb.AuthOverrides do
     set :root_class, "md:min-w-md"
   end
 
-  # Replace banner logo with text
+  # Replace banner logo with text (no image in light or dark so link has discernible text)
   override AshAuthentication.Phoenix.Components.Banner do
     set :text, "Mitgliederverwaltung"
     set :image_url, nil
+    set :dark_image_url, nil
   end
 
   # Translate the "or" in the horizontal rule (between password form and SSO).