Describe the bug
After upgrading from Deck 1.15.2 to 1.15.3, user mentions (@username) in comments are completely broken.

To Reproduce

1. Open any Deck card
2. Go to Comments tab
3. Try to type @ to mention a user
4. Autocomplete doesn't appear

Browser Console Errors:
TypeError: Cannot read properties of undefined (reading 'toLowerCase')
at CommentForm.vue:1928:3

TypeError: Cannot read properties of undefined (reading 'data-mention-id')
at CommentForm.vue:1921:1

Environment:

• Nextcloud: 31.0.9.1
• Deck: 1.15.3
• Browser: Chrome/Firefox
• OS: Ubuntu 24.04

Additional context
Downgrade to 1.15.2 fixes the issue immediately.