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/src/pages/auth/reset-password.vue

@@ -26,7 +26,7 @@
           ref="turnstile"
           v-model="token"
           class="turnstile"
-          :options="{ theme: $colorMode.value === 'light' ? 'light' : 'dark' }"
+          :options="{ theme: $theme.active === 'light' ? 'light' : 'dark' }"
         />
 
         <button class="btn btn-primary centered-btn" :disabled="!token" @click="recovery">

apps/frontend/src/pages/auth/sign-in.vue

@@ -85,7 +85,7 @@
           ref="turnstile"
           v-model="token"
           class="turnstile"
-          :options="{ theme: $colorMode.value === 'light' ? 'light' : 'dark' }"
+          :options="{ theme: $theme.active === 'light' ? 'light' : 'dark' }"
         />
 
         <button

apps/frontend/src/pages/auth/sign-up.vue

@@ -110,7 +110,7 @@
         ref="turnstile"
         v-model="token"
         class="turnstile"
-        :options="{ theme: $colorMode.value === 'light' ? 'light' : 'dark' }"
+        :options="{ theme: $theme.active === 'light' ? 'light' : 'dark' }"
       />
 
       <button