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/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)