chore: Improve translation service with HTML and plain text support (#11305)

# Pull Request Template

## Description

This PR changes to translation to properly handle different content
types during translation.

### Changes

1. **Email translation with HTML support**
   - Properly detects and preserves HTML content from emails
   - Sets `mime_type` to 'text/html' when HTML content is present

2. **Email translation with plain text support**
   - Falls back to email text content when HTML is not available
- Sets `mime_type` to 'text/plain' when HTML is not available and
content type includes 'text/plain'

3. **Plain message with plain text support (Non email channels)**
   - Sets `mime_type` to 'text/plain' for non-email channels
- Fixes an issue where Markdown formatting was being lost due to
incorrect `mime_type`
   
**Note**: Translation for very long emails is not currently supported.

Fixes
https://linear.app/chatwoot/issue/CW-4244/translate-button-doesnt-work-in-email-channels

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

**Loom video**

https://www.loom.com/share/8f8428ed2cfe415ea5cb6c547c070f00?sid=eab9fa11-05f8-4838-9181-334bee1023c4


## 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

app/javascript/dashboard/composables/useTranslations.js

@@ -0,0 +1,22 @@
+import { computed } from 'vue';
+
+/**
+ * Composable to extract translation state/content from contentAttributes.
+ * @param {Ref|Reactive} contentAttributes - Ref or reactive object containing `translations` property
+ * @returns {Object} { hasTranslations, translationContent }
+ */
+export function useTranslations(contentAttributes) {
+  const hasTranslations = computed(() => {
+    if (!contentAttributes.value) return false;
+    const { translations = {} } = contentAttributes.value;
+    return Object.keys(translations || {}).length > 0;
+  });
+
+  const translationContent = computed(() => {
+    if (!hasTranslations.value) return null;
+    const translations = contentAttributes.value.translations;
+    return translations[Object.keys(translations)[0]];
+  });
+
+  return { hasTranslations, translationContent };
+}