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/components/AgentMessageBubble.vue

@@ -6,7 +6,7 @@ import ChatOptions from 'shared/components/ChatOptions.vue';
 import ChatArticle from './template/Article.vue';
 import EmailInput from './template/EmailInput.vue';
 import CustomerSatisfaction from 'shared/components/CustomerSatisfaction.vue';
-import darkModeMixin from 'widget/mixins/darkModeMixin.js';
+import { useDarkMode } from 'widget/composables/useDarkMode';
 import IntegrationCard from './template/IntegrationCard.vue';
 
 export default {
@@ -20,7 +20,6 @@ export default {
     CustomerSatisfaction,
     IntegrationCard,
   },
-  mixins: [darkModeMixin],
   props: {
     message: { type: String, default: null },
     contentType: { type: String, default: null },
@@ -34,11 +33,13 @@ export default {
   setup() {
     const { formatMessage, getPlainText, truncateMessage, highlightContent } =
       useMessageFormatter();
+    const { getThemeClass } = useDarkMode();
     return {
       formatMessage,
       getPlainText,
       truncateMessage,
       highlightContent,
+      getThemeClass,
     };
   },
   computed: {
@@ -98,7 +99,7 @@ export default {
         !isCards && !isOptions && !isForm && !isArticle && !isCards && !isCSAT
       "
       class="chat-bubble agent"
-      :class="$dm('bg-white', 'dark:bg-slate-700 has-dark-mode')"
+      :class="getThemeClass('bg-white', 'dark:bg-slate-700 has-dark-mode')"
     >
       <div
         v-dompurify-html="formatMessage(message, false)"