feat: Allow removing labels via conversation context menu (#13525)

# Pull Request Template

## Description

This PR adds support for removing labels from the conversation card
context menu. Assigned labels now show a checkmark, and clicking an
already-selected label will remove it.

Fixes
https://linear.app/chatwoot/issue/CW-6400/allow-removing-labels-directly-from-the-right-click-menu
https://github.com/chatwoot/chatwoot/issues/13367
## Type of change

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

## How Has This Been Tested?

**Screencast**


https://github.com/user-attachments/assets/4e3a6080-a67d-4851-9d10-d8dbf3ceeb04




## 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
- [ ] 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/i18n/locale/en/conversation.json

@@ -174,6 +174,10 @@
           "SUCCESFUL": "Assigned label #{labelName} to conversation id {conversationId}",
           "FAILED": "Couldn't assign label. Please try again."
         },
+        "LABEL_REMOVAL": {
+          "SUCCESFUL": "Removed label #{labelName} from conversation id {conversationId}",
+          "FAILED": "Couldn't remove label. Please try again."
+        },
         "TEAM_ASSIGNMENT": {
           "SUCCESFUL": "Assigned team \"{team}\" to conversation id {conversationId}",
           "FAILED": "Couldn't assign team. Please try again."