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/shared/components/DateSeparator.vue

@@ -1,15 +1,18 @@
 <script>
 import { formatDate } from 'shared/helpers/DateHelper';
-import darkModeMixin from 'widget/mixins/darkModeMixin.js';
+import { useDarkMode } from 'widget/composables/useDarkMode';
 
 export default {
-  mixins: [darkModeMixin],
   props: {
     date: {
       type: String,
       required: true,
     },
   },
+  setup() {
+    const { getThemeClass } = useDarkMode();
+    return { getThemeClass };
+  },
   computed: {
     formattedDate() {
       return formatDate({
@@ -25,7 +28,7 @@ export default {
 <template>
   <div
     class="date--separator"
-    :class="$dm('text-slate-700', 'dark:text-slate-200')"
+    :class="getThemeClass('text-slate-700', 'dark:text-slate-200')"
   >
     {{ formattedDate }}
   </div>