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/caldav/backend_test.go

@@ -245,7 +245,7 @@ func TestPropFindExplicitCalendarColorRequest(t *testing.T) {
 	if err := dbase.CreateCalendarWithColor("alice", "work", "#3b82f6"); err != nil {
 		t.Fatalf("CreateCalendarWithColor: %v", err)
 	}
-	if err := st.EnsureCollection("alice", "cal-work"); err != nil {
+	if err := st.EnsureCollection("alice", "calendars/work"); err != nil {
 		t.Fatalf("EnsureCollection: %v", err)
 	}