This change implements the planned @mention feature for calendar notes.
It includes:

1. Database schema updates to store user emails.
2. Server actions to sync authenticated users and search for mentions.
3. Parsing logic to extract @mentions and store them as validated User IDs.
4. UI enhancements for autocomplete and visual tagging in the notepad.

PR created automatically by Jules for task 5204406483989686825 started by @ngoiyaeric

Summary by CodeRabbit

Release Notes

• New Features

  • Added @mention support for tagging users in calendar notes with user suggestion dropdown.
  • Introduced location tagging in notes with automatic map marker synchronization.
  • Implemented real-time note updates across all active sessions.
  • Added user search functionality.

• Improvements

  • Enhanced calendar note display with user indicators, timestamps, and improved layouts.
  • Refined database schema with improved user data management.