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/pages/app.vue

@@ -896,7 +896,7 @@ useSeoMeta({
     </div>
     <div class="logo-banner">
       <svg
-        v-if="$colorMode.value === 'light'"
+        v-if="$theme.active === 'light'"
         viewBox="0 0 865 512"
         fill="none"
         xmlns="http://www.w3.org/2000/svg"