Make the token-store file lock compile on Windows

tokenstore.go used syscall.Flock and syscall.LOCK_* directly, which don't exist on Windows — so the public library (which transitively imports internal/auth) failed to cross-compile for GOOS=windows.

Split the lock behind build tags: tokenstore_lock_unix.go keeps the real flock(2), and tokenstore_lock_windows.go provides a compiling fallback. Windows has no flock; the file store's atomic temp-file+rename keeps individual writes safe, and the lock only guarded the rare concurrent read-modify-write lost update, so the fallback opens the lock file as a no-op advisory lock rather than pulling in a Windows locking dependency.

Verified with GOOS=windows go build/vet ./... (previously failed to compile).