feat: implement unified directory structure with automatic migration for CalDAV and CardDAV

- Unify directory structure across protocols:
  * WebDAV: data/files/<username>/
  * CalDAV: data/<username>/calendars/<name>/
  * CardDAV: data/<username>/addressbooks/<name>/

- Add automatic migration capability that runs on server startup
- Maintain full backward compatibility with existing installations
- Improve Docker usage by automatically handling legacy data structure

- Updated storage provider implementations to use new nested structure
- Enhanced store functions for backward compatibility
- Modified CalDAV and CardDAV backends to use unified paths
- Added automatic migration logic in server initialization

internal/db/users.go

@@ -102,8 +102,6 @@ func (d *DB) DisplayName(username string) string {
 	return u.DisplayName
 }
 
-
-
 // DeleteUser removes username along with all of its calendars, address
 // books, and sharing grants (calendars/addressbooks cascade via foreign
 // key; shares are cleaned up explicitly since they reference usernames as