scss: migrate @import -> @use, lighten/darken -> color.adjust (dart-sass)

Silences the dart-sass deprecation warnings that flooded every build:
- All @import of local partials (colors, mixins, and the CSS-emitting
  partials) -> @use '...' as * (partials are pure defs, so 'as *' keeps the
  global names and the ~25 call sites unchanged). Added explicit @use './colors'
  to bootstrap-overrides.scss and cookies-eu.scss, which used -palette*
  transitively (broke under @use's scoping).
- lighten(c, x%) -> color.adjust(c, $lightness: x%) and darken -> negative,
  with @use 'sass:color' in the 6 affected files.
Compiles clean (byte-for-byte CSS behavior preserved).

imports/ui/components/OAuthLoginButton/OAuthLoginButton.scss

@@ -1,4 +1,5 @@
-@import '../../stylesheets/colors';
+@use 'sass:color';
+@use '../../stylesheets/colors' as *;
 
 .OAuthLoginButton {
 /*  display: block;
@@ -19,21 +20,21 @@
     background: $facebook;
     color: #fff;
 
-    &:hover { background: darken($facebook, 5%); }
+    &:hover { background: color.adjust($facebook, $lightness: -5%); }
   }
 
   &.OAuthLoginButton-google {
     background: $google;
     color: #fff;
 
-    &:hover { background: darken($google, 5%); }
+    &:hover { background: color.adjust($google, $lightness: -5%); }
   }
 
   &.OAuthLoginButton-github {
     background: $github;
     color: #fff;
 
-    &:hover { background: darken($github, 5%); }
+    &:hover { background: color.adjust($github, $lightness: -5%); }
   }
 
   &:active {