Enable @mentions in comments and posts with user search autocomplete.

Acceptance Criteria:

• Detect @-prefix in text editors
• Show autocomplete dropdown with matching usernames
• Search by username and display name
• Show avatar and name in suggestions (limit 5)
• Parse mentions on submit and store in MentionModel
• Render mentions as styled links to user profiles
• Generate notifications for mentioned users
• Handle username changes gracefully (store user ID, not username)

Technical Notes:

• Use debounced API call for search (300ms)
• Store mention positions for highlighting in rendered content