Rewrite cosmetics and theme preferences (#1292)

- Cosmetics and theme preferences are now only stored in cookies instead
  of a combination of both cookies and state.

- The theme plugin now supports client hints. This allows the server
  to render a page using the client-preferred theme provided it supplies
  this information (any browser other than Firefox), helping to avoid an
  annoying flash while the page is hydrating.

- The previous workaround using the Nitro plugin has been removed. Its
  functionality is now handled by the Nuxt theme plugin with cleaner
  code.

- All pages and components now use the new plugins.

apps/frontend/src/composables/nuxt-accessors.ts

@@ -0,0 +1,7 @@
+export function useTheme() {
+  return useNuxtApp().$theme;
+}
+
+export function useCosmetics() {
+  return useNuxtApp().$cosmetics;
+}