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/stylesheets/app.scss

@@ -1,6 +1,6 @@
-@import './colors';
-@import './forms';
-@import './bootstrap-overrides';
-@import './custom';
-@import './cookies-eu';
-@import './new-age';
+@use './colors' as *;
+@use './forms' as *;
+@use './bootstrap-overrides' as *;
+@use './custom' as *;
+@use './cookies-eu' as *;
+@use './new-age' as *;

imports/ui/stylesheets/bootstrap-overrides.scss

@@ -1,7 +1,9 @@
-@import './mixins';
+@use 'sass:color';
+@use './mixins' as *;
+@use './colors' as *;
 
 body {
-  background-color: lighten($todos-palette5, 70%);
+  background-color: color.adjust($todos-palette5, $lightness: 70%);
   /* for fixed header */
   /* padding-top: 50px; */
   overflow-x: hidden;

imports/ui/stylesheets/cookies-eu.scss

@@ -1,5 +1,7 @@
+@use 'sass:color';
+@use './colors' as *;
 #js-cookie-policy-default {
-  background-color: lighten($todos-palette3, 10%);
+  background-color: color.adjust($todos-palette3, $lightness: 10%);
 }
 
 .cookie-policy {

imports/ui/stylesheets/custom.scss

@@ -1,5 +1,5 @@
-@import './mixins';
-@import './colors';
+@use './mixins' as *;
+@use './colors' as *;
 
 a, a:hover  {
   color: $todos-palette1a;

imports/ui/stylesheets/forms.scss

@@ -1,5 +1,5 @@
-@import './mixins';
-@import './colors';
+@use './mixins' as *;
+@use './colors' as *;
 
 form {
   label,

imports/ui/stylesheets/new-age.scss

@@ -1,4 +1,4 @@
-@import './mixins';
+@use './mixins' as *;
 
 /*!
  * Start Bootstrap - New Age v4.0.0-beta.2 (https://startbootstrap.com/template-overviews/new-age)