fix: use slash-separated lock paths on Windows - #344

Summary

• build shared lock names with slash separators on every platform
• add regression coverage for root locks and nested user directories
• run the test workflow on Windows and make the existing path/filename assertions portable

Problem

lockSystem adapts filesystem-backed users into one shared webdav.LockSystem namespace. It currently uses filepath.Join for that namespace.

The golang.org/x/net/webdav.LockSystem contract requires resource names to use / separators regardless of the host OS. On Windows, an infinite-depth lock is stored under a name such as /D:\data, while a descendant is resolved as /D:\data\file.txt. The in-memory lock system therefore does not recognize the latter as a descendant of the former.

This is observable in TestServerPartialUpdateHonorsLocks/root: a tokenless partial PATCH to a descendant returned 204 No Content instead of 423 Locked.

Fix

Normalize the filesystem directory with filepath.ToSlash, then join it to the WebDAV resource name with path.Join. This preserves the backing-directory namespace while satisfying the platform-independent LockSystem path contract.

The new tests cover both direct root-lock inheritance and the shared namespace used when one user's directory is nested under another user's directory.

Testing

Run on Windows:

• go test ./...
• go build ./...
• go vet ./...

The test workflow now covers both ubuntu-latest (with the race detector) and windows-latest.

Coordination

#335 also extends locksystem.go for multi-directory support. This PR only fixes the current single-directory path adapter. If #335 lands first, I can rebase this focused change and carry the same slash-separated invariant into its added resolver.