Migrate Sass files to module system

styles/_containers.scss

@@ -1,3 +1,5 @@
+@use 'mixins' as *;
+
 // Core containers
 
 body {

styles/_dashboard.scss

@@ -1,3 +1,5 @@
+@use 'mixins' as *;
+
 #game-dashboard {
   width: 230px;
   // Prevent grid from shifting awkwardly when game controls disappear/reappear

styles/_grid.scss

@@ -1,3 +1,5 @@
+@use 'mixins' as *;
+
 // The game grid, including all of its layers as well as individual chips
 
 #grid {

styles/_header.scss

@@ -1,3 +1,5 @@
+@use 'mixins' as *;
+
 // Elements displayed in the topmost region of the app
 
 h1 {

styles/_player-area.scss

@@ -1,3 +1,5 @@
+@use 'mixins' as *;
+
 #player-area {
   position: relative;
 }

styles/index.scss

@@ -1,13 +1,13 @@
 // Main front-end stylesheet
 
-@import 'mixins';
-@import 'variables';
-@import 'colors';
-@import 'containers';
-@import 'header';
-@import 'dashboard';
-@import 'dashboard-controls';
-@import 'grid';
-@import 'player-area';
-@import 'reaction-picker';
-@import 'update-notification';
+@use 'mixins';
+@use 'variables';
+@use 'colors';
+@use 'containers';
+@use 'header';
+@use 'dashboard';
+@use 'dashboard-controls';
+@use 'grid';
+@use 'player-area';
+@use 'reaction-picker';
+@use 'update-notification';