feat: Add rich text support for widget welcome tagline (#11666)

# Pull Request Template

## Description

This PR adds rich text support for the widget welcome tagline, enabling
users to format the message using basic text styling (bold, italics,
links, etc.)
https://github.com/chatwoot/chatwoot/pull/11629#issuecomment-2932448975

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

### Screenshots
<img width="1100" alt="image"
src="https://github.com/user-attachments/assets/eef2bfd3-7bc9-4aea-b21d-078f6b29f334"
/>
<img width="448" alt="image"
src="https://github.com/user-attachments/assets/713c6b07-d0dc-4c8f-bfba-cd119a858375"
/>



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] 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
- [ ] 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

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/widget/components/ChatHeaderExpanded.vue

@@ -1,6 +1,7 @@
 <script setup>
 import HeaderActions from './HeaderActions.vue';
 import { computed } from 'vue';
+import { useMessageFormatter } from 'shared/composables/useMessageFormatter';
 
 const props = defineProps({
   avatarUrl: {
@@ -21,6 +22,8 @@ const props = defineProps({
   },
 });
 
+const { formatMessage } = useMessageFormatter();
+
 const containerClasses = computed(() => [
   props.avatarUrl ? 'justify-between' : 'justify-end',
 ]);
@@ -47,8 +50,8 @@ const containerClasses = computed(() => [
       class="mt-4 text-2xl mb-1.5 font-medium text-n-slate-12"
     />
     <p
-      v-dompurify-html="introBody"
-      class="text-lg leading-normal text-n-slate-11"
+      v-dompurify-html="formatMessage(introBody)"
+      class="text-lg leading-normal text-n-slate-11 [&_a]:underline"
     />
   </header>
 </template>