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/plugins/1.theme.js

@@ -1,27 +0,0 @@
-export default defineNuxtPlugin(async (nuxtApp) => {
-  await useAuth();
-  await useUser();
-  const themeStore = useTheme();
-  const cosmetics = useCosmetics();
-
-  nuxtApp.hook("app:mounted", () => {
-    if (import.meta.client && themeStore.value.preference === "system") {
-      const colorSchemeQueryList = window.matchMedia("(prefers-color-scheme: light)");
-
-      const setColorScheme = (e) => {
-        if (themeStore.value.preference === "system") {
-          if (e.matches) {
-            updateTheme("light");
-          } else {
-            updateTheme(cosmetics.value.preferredDarkTheme ?? "dark");
-          }
-        }
-      };
-
-      setColorScheme(colorSchemeQueryList);
-      colorSchemeQueryList.addEventListener("change", setColorScheme);
-    }
-  });
-
-  nuxtApp.provide("colorMode", themeStore.value);
-});