💄 Use of border-box box sizing

This makes calculating the size of the UI components easier, especially those which have padding, margin or border.

src/ui/components/_panel.scss

@@ -5,8 +5,9 @@ $panel-aspect-ratio: 1/1.618;
 @mixin panel($padding-v: 2%, $padding-h: 2%) {
     padding: $padding-v $padding-h;
 
-    height: calc(100% - (2 * $padding-v) - (2 * $border-big-width));
-    width: calc(100% - (2 * $padding-h) - (2 * $border-big-width));
+    height: 100%;
+    width: 100%;
+
     flex-shrink: 0;
 
     border: $border-big;