Add folder permissions UI and WebDAV protocol support
Permissions UI:
- FolderPermissionsDialog component with public/private toggle,
role/user permission management, and access level badges
- Integrated into file manager toolbar (visible for folder admins)
- Backend returns accessLevel in folder detail endpoint
WebDAV server:
- Full WebDAV protocol at /webdav/ with Basic Auth (existing credentials)
- PROPFIND, GET, PUT, DELETE, MKCOL, COPY, MOVE, LOCK/UNLOCK support
- Permission-checked against existing folder access model
- In-memory lock stubs for Windows client compatibility
- 22 API integration tests covering all operations
Also fixes canAccess to check folder creator (was missing).
51ca2ca683