chore: Replace darkmode mixin with useDarkMode composable [CW-3474] (#9949)

# Pull Request Template

## Description

Replaces darkModeMixin with the new useDarkMode composable and replaces
wll usages of mixin the the composable in components and pages

Fixes
https://linear.app/chatwoot/issue/CW-3474/rewrite-darkmodemixin-mixin-to-a-composable

## Type of change

Please delete options that are not relevant.

- [x] New feature (non-breaking change which adds functionality)

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Sivin Varghese <64252451+iamsivin@users.noreply.github.com>

app/javascript/widget/views/Home.vue

@@ -4,7 +4,7 @@ import ArticleHero from 'widget/components/ArticleHero.vue';
 import ArticleCardSkeletonLoader from 'widget/components/ArticleCardSkeletonLoader.vue';
 
 import { mapGetters } from 'vuex';
-import darkModeMixin from 'widget/mixins/darkModeMixin';
+import { useDarkMode } from 'widget/composables/useDarkMode';
 import routerMixin from 'widget/mixins/routerMixin';
 import configMixin from 'widget/mixins/configMixin';
 
@@ -15,7 +15,11 @@ export default {
     TeamAvailability,
     ArticleCardSkeletonLoader,
   },
-  mixins: [configMixin, routerMixin, darkModeMixin],
+  mixins: [configMixin, routerMixin],
+  setup() {
+    const { prefersDarkMode } = useDarkMode();
+    return { prefersDarkMode };
+  },
   computed: {
     ...mapGetters({
       availableAgents: 'agent/availableAgents',