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/vue.ts

@@ -0,0 +1,14 @@
+/**
+ * Creates a computed reference that uses a provide getter function called with an argument representing the current mount state of the component.
+ * @param getter A getter function that will run with `mounted` argument representing whether or not the component is mounted.
+ * @returns A computed reference that changes when component becomes mounted or unmounted.
+ */
+export function useMountedValue<T>(getter: (isMounted: boolean) => T) {
+  const mounted = ref(getCurrentInstance()?.isMounted ?? false);
+
+  onMounted(() => (mounted.value = true));
+
+  onUnmounted(() => (mounted.value = false));
+
+  return computed(() => getter(mounted.value));
+}