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/specs/DateSeparator.spec.js

@@ -2,7 +2,6 @@ import DateSeparator from '../DateSeparator.vue';
 import { createLocalVue, shallowMount } from '@vue/test-utils';
 import Vuex from 'vuex';
 import VueI18n from 'vue-i18n';
-import darkModeMixin from 'widget/mixins/darkModeMixin.js';
 const localVue = createLocalVue();
 import i18n from 'dashboard/i18n';
 localVue.use(Vuex);
@@ -40,7 +39,6 @@ describe('dateSeparator', () => {
       propsData: { date: 'Nov 18, 2019' },
       mocks: { $t: msg => msg },
       i18n: i18nConfig,
-      mixins: [darkModeMixin],
     });
   });