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/App.vue

@@ -18,7 +18,7 @@ import {
   ON_CAMPAIGN_MESSAGE_CLICK,
   ON_UNREAD_MESSAGE_CLICK,
 } from './constants/widgetBusEvents';
-import darkModeMixin from 'widget/mixins/darkModeMixin';
+import { useDarkMode } from 'widget/composables/useDarkMode';
 import { SDK_SET_BUBBLE_VISIBILITY } from '../shared/constants/sharedFrameEvents';
 import { emitter } from 'shared/helpers/mitt';
 
@@ -27,7 +27,11 @@ export default {
   components: {
     Spinner,
   },
-  mixins: [availabilityMixin, configMixin, routerMixin, darkModeMixin],
+  mixins: [availabilityMixin, configMixin, routerMixin],
+  setup() {
+    const { prefersDarkMode } = useDarkMode();
+    return { prefersDarkMode };
+  },
   data() {
     return {
       isMobile: false,