refactor: unify data directory structure

Unify user data directory from fragmented layout to consistent nested format:

- Move WebDAV from: data/files/<username> → data/<username>/files/
- Move CalDAV from: data/<username>/cal-<name> → data/<username>/calendars/<name>
- Move CardDAV from: data/<username>/card-<name> → data/<username>/addressbooks/<name>

Changes:
- internal/store/store.go: Update collectionPath() to map collection names
- internal/store/migrate.go: Add idempotent Migrate() method
- internal/store/migrate_test.go: Comprehensive migration tests
- internal/webdav/handler.go: Use new unified path structure
- cmd/server/main.go: Auto-run migration on startup
- tools/nidusctl/main.go: Add migrate subcommand
- Update tests to verify new structure

URL endpoints unchanged - only on-disk structure modified. All tests pass.

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