feat: Migrate availability mixins to composable and helper (#11596)

# Pull Request Template

## Description

**This PR includes:**

* Refactored two legacy mixins (`availability.js`,
`nextAvailability.js`) into a Vue 3 composable (`useAvailability`),
helper module and component based rendering logic.
* Fixed an issue where the widget wouldn't load if business hours were
enabled but all days were unchecked.
* Fixed translation issue
[[#11280](https://github.com/chatwoot/chatwoot/issues/11280)](https://github.com/chatwoot/chatwoot/issues/11280).
* Reduced code complexity and size.
* Added test coverage for both the composable and helper functions.

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

### Loom video

https://www.loom.com/share/2bc3ed694b4349419505e275d14d0b98?sid=22d585e4-0dc7-4242-bcb6-e3edc16e3aee

### Story
<img width="995" height="442" alt="image"
src="https://github.com/user-attachments/assets/d6340738-07db-41d5-86fa-a8ecf734cc70"
/>



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules


Fixes https://github.com/chatwoot/chatwoot/issues/12012

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>
Co-authored-by: Pranav <pranav@chatwoot.com>
Co-authored-by: Shivam Mishra <scm.mymail@gmail.com>

app/javascript/histoire.setup.ts

@@ -1,6 +1,7 @@
 import './design-system/histoire.scss';
 import { defineSetupVue3 } from '@histoire/plugin-vue';
-import i18nMessages from 'dashboard/i18n';
+import dashboardI18n from 'dashboard/i18n';
+import widgetI18n from 'widget/i18n';
 import { createI18n } from 'vue-i18n';
 import { vResizeObserver } from '@vueuse/components';
 import store from 'dashboard/store';
@@ -9,10 +10,30 @@ import VueDOMPurifyHTML from 'vue-dompurify-html';
 import { domPurifyConfig } from 'shared/helpers/HTMLSanitizer.js';
 import { directive as onClickaway } from 'vue3-click-away';
 
+function mergeMessages(...sources) {
+  return sources.reduce((acc, src) => {
+    Object.keys(src).forEach(key => {
+      if (
+        acc[key] &&
+        typeof acc[key] === 'object' &&
+        typeof src[key] === 'object'
+      ) {
+        acc[key] = mergeMessages(acc[key], src[key]);
+      } else {
+        acc[key] = src[key];
+      }
+    });
+    return acc;
+  }, {});
+}
+
 const i18n = createI18n({
   legacy: false, // https://github.com/intlify/vue-i18n/issues/1902
   locale: 'en',
-  messages: i18nMessages,
+  messages: mergeMessages(
+    structuredClone(dashboardI18n),
+    structuredClone(widgetI18n)
+  ),
 });
 
 export const setupVue3 = defineSetupVue3(({ app }) => {