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

- Unify directory structure across all protocols under user directory:
  * WebDAV: data/<username>/files/
  * 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
- Changed WebDAV path from data/files/<username>/ to data/<username>/files/

internal/webdav/handler.go

@@ -14,7 +14,7 @@ import (
 
 // NewHandler returns an http.Handler that provides standard WebDAV file access,
 // mounted at the fixed URL /files/ for every user and rooted at
-// dataDir/files/<username>/ on disk. The URL is the same for all users —
+// dataDir/<username>/files/ on disk. The URL is the same for all users —
 // which user's directory is served is resolved from the Basic Auth identity
 // in the request context, not from the URL.
 //
@@ -38,7 +38,7 @@ func NewHandler(cfg *config.Config, dataDir string, logger *slog.Logger) http.Ha
 		h, ok := handlers[p.Username]
 		if !ok {
 			username := p.Username
-			userDir := filepath.Join(dataDir, "files", username)
+			userDir := filepath.Join(dataDir, username, "files")
 			if err := os.MkdirAll(userDir, 0o755); err != nil {
 				mu.Unlock()
 				logger.Error("creating user WebDAV dir", "user", username, "error", err)