Rewrite cosmetics and theme preferences (#1328)

- 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.

- For the future, the theme plugin now supports additional light themes.
  Light theme preferences are currently not stored, but this can easily
  be fixed if the need arises.

- 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.

- Compared to the previous attempt, this commit has been improved to be
  more robust in cases where the theme cookie contains invalid values.

Co-authored-by: Geometrically <18202329+Geometrically@users.noreply.github.com>

apps/frontend/nuxt.config.ts

@@ -408,6 +408,14 @@ export default defineNuxtConfig({
       autoprefixer: {},
     },
   },
+  routeRules: {
+    "/**": {
+      headers: {
+        "Accept-CH": "Sec-CH-Prefers-Color-Scheme",
+        "Critical-CH": "Sec-CH-Prefers-Color-Scheme",
+      },
+    },
+  },
   compatibilityDate: "2024-07-03",
 });