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/mixins/darkModeMixin.js

@@ -1,24 +0,0 @@
-import { mapGetters } from 'vuex';
-
-export default {
-  computed: {
-    ...mapGetters({ darkMode: 'appConfig/darkMode' }),
-    prefersDarkMode() {
-      const isOSOnDarkMode =
-        this.darkMode === 'auto' &&
-        window.matchMedia('(prefers-color-scheme: dark)').matches;
-      return isOSOnDarkMode || this.darkMode === 'dark';
-    },
-  },
-  methods: {
-    $dm(light, dark) {
-      if (this.darkMode === 'light') {
-        return light;
-      }
-      if (this.darkMode === 'dark') {
-        return dark;
-      }
-      return light + ' ' + dark;
-    },
-  },
-};

app/javascript/widget/mixins/specs/darkModeMixin.spec.js

@@ -1,41 +0,0 @@
-import { shallowMount, createLocalVue } from '@vue/test-utils';
-import darkModeMixin from '../darkModeMixin';
-import Vuex from 'vuex';
-const localVue = createLocalVue();
-localVue.use(Vuex);
-
-const darkModeValues = ['light', 'auto'];
-
-describe('darkModeMixin', () => {
-  let getters;
-  let store;
-  beforeEach(() => {
-    getters = {
-      'appConfig/darkMode': () => darkModeValues[0],
-    };
-    store = new Vuex.Store({ getters });
-  });
-
-  it('if light theme', () => {
-    const Component = {
-      render() {},
-      mixins: [darkModeMixin],
-    };
-    const wrapper = shallowMount(Component, { store, localVue });
-    expect(wrapper.vm.$dm('bg-100', 'bg-600')).toBe('bg-100');
-  });
-
-  it('if auto theme', () => {
-    getters = {
-      'appConfig/darkMode': () => darkModeValues[2],
-    };
-    store = new Vuex.Store({ getters });
-
-    const Component = {
-      render() {},
-      mixins: [darkModeMixin],
-    };
-    const wrapper = shallowMount(Component, { store, localVue });
-    expect(wrapper.vm.$dm('bg-100', 'bg-600')).toBe('bg-100 bg-600');
-  });
-});