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/layouts/default.vue

@@ -62,7 +62,7 @@
                 :title="formatMessage(messages.changeTheme)"
                 @click="changeTheme"
               >
-                <MoonIcon v-if="$colorMode.value === 'light'" aria-hidden="true" />
+                <MoonIcon v-if="$theme.active === 'light'" aria-hidden="true" />
                 <SunIcon v-else aria-hidden="true" />
               </button>
               <div
@@ -242,7 +242,7 @@
               {{ formatMessage(commonMessages.settingsLabel) }}
             </NuxtLink>
             <button class="iconified-button" @click="changeTheme">
-              <MoonIcon v-if="$colorMode.value === 'light'" class="icon" />
+              <MoonIcon v-if="$theme.active === 'light'" class="icon" />
               <SunIcon v-else class="icon" />
               <span class="dropdown-item__text">
                 {{ formatMessage(messages.changeTheme) }}
@@ -403,7 +403,7 @@
           {{ formatMessage(messages.getModrinthApp) }}
         </nuxt-link>
         <button class="iconified-button raised-button" @click="changeTheme">
-          <MoonIcon v-if="$colorMode.value === 'light'" aria-hidden="true" />
+          <MoonIcon v-if="$theme.active === 'light'" aria-hidden="true" />
           <SunIcon v-else aria-hidden="true" />
           {{ formatMessage(messages.changeTheme) }}
         </button>
@@ -449,7 +449,6 @@ import ModalCreation from "~/components/ui/ModalCreation.vue";
 import Avatar from "~/components/ui/Avatar.vue";
 import { getProjectTypeMessage } from "~/utils/i18n-project-type.ts";
 import { commonMessages } from "~/utils/common-messages.ts";
-import { DARK_THEMES } from "~/composables/theme.js";
 
 const { formatMessage } = useVIntl();
 
@@ -738,18 +737,11 @@ function toggleBrowseMenu() {
     isMobileMenuOpen.value = false;
   }
 }
-function changeTheme() {
-  updateTheme(
-    DARK_THEMES.includes(app.$colorMode.value)
-      ? "light"
-      : cosmetics.value.preferredDarkTheme ?? "dark",
-    true,
-  );
-}
+
+const { cycle: changeTheme } = useTheme();
 
 function hideStagingBanner() {
   cosmetics.value.hideStagingBanner = true;
-  saveCosmetics();
 }
 </script>