feat: convert templates from njk to html

Note: Nunjucks will still process the templates, but this way they can
be plain HTML files and will be formatted properly.

src/assets/css/global/variables.css

@@ -0,0 +1,5 @@
+/* Global variables */
+:root {
+  --transition-duration: 250ms;
+  --transition-timing: ease;
+}