Add VirtualFs tag-mode WebDAV + MyFiles UI + Admin WebDAV endpoint

- VirtualFs: SQLite-backed virtual folders (tag mode), 16 unit tests
- MyFiles module: API endpoints + Web UI for folder/tag management
- Admin WebDAV: /admin-webdav/*path with Basic Auth + URI prefix rewrite
- CLI: webdav-folder/tag/untag/list/start --virtual-mode commands
- Deployed and tested on M5Max48: PROPFIND, PUT, GET, DELETE all working

markbase-core/src/vfs/mod.rs

@@ -9,6 +9,7 @@ pub mod smb_fs;
 #[cfg(feature = "smb-server")]
 pub mod smb_server_backend;
 pub mod util;
+pub mod virtual_fs;
 #[cfg(feature = "async-vfs")]
 pub mod async_fs;
 #[cfg(feature = "async-vfs")]