chore: added a new sort by unread option for conversations (CW-7152) (#14512)

## Description

Added a new option for the sort by option in conversation filters called
unread. This is to filter out unread conversations.
Fixes # CW-7152

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

Tested manually in local.

<img width="1397" height="603" alt="Screenshot 2026-05-20 at 10 40
20 PM"
src="https://github.com/user-attachments/assets/6c60263e-907b-419f-a7ed-06010bbe8736"
/>


## 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
- [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/constants/globals.js

@@ -27,6 +27,7 @@ export default {
     WAITING_SINCE_ASC: 'waiting_since_asc',
     WAITING_SINCE_DESC: 'waiting_since_desc',
     PRIORITY_DESC_CREATED_AT_ASC: 'priority_desc_created_at_asc',
+    UNREAD: 'unread',
   },
   ARTICLE_STATUS_TYPES: {
     DRAFT: 0,