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/web/calendar.go

@@ -527,7 +527,6 @@ func mondayOf(t time.Time) time.Time {
 	return time.Date(t.Year(), t.Month(), t.Day(), 0, 0, 0, 0, t.Location()).AddDate(0, 0, -offset)
 }
 
-
 // eventDayRange returns the inclusive [start, end] calendar-day span an
 // event occupies, in loc, for placing it on the month grid.
 func eventDayRange(form templates.EventFormData, loc *time.Location) (start, end time.Time, err error) {