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/FileBubble.vue

@@ -1,13 +1,12 @@
 <script>
 import FluentIcon from 'shared/components/FluentIcon/Index.vue';
-import darkModeMixin from 'widget/mixins/darkModeMixin.js';
+import { useDarkMode } from 'widget/composables/useDarkMode';
 import { getContrastingTextColor } from '@chatwoot/utils';
 
 export default {
   components: {
     FluentIcon,
   },
-  mixins: [darkModeMixin],
   props: {
     url: {
       type: String,
@@ -26,6 +25,10 @@ export default {
       default: false,
     },
   },
+  setup() {
+    const { getThemeClass } = useDarkMode();
+    return { getThemeClass };
+  },
   computed: {
     title() {
       return this.isInProgress
@@ -45,7 +48,7 @@ export default {
     },
     titleColor() {
       return !this.isUserBubble
-        ? this.$dm('text-black-900', 'dark:text-slate-50')
+        ? this.getThemeClass('text-black-900', 'dark:text-slate-50')
         : '';
     },
   },