headscale 0.28 added GET /v1/preauthkey without a user param
which returns all keys including tag-only ones. this change
adds getAllPreAuthKeys() and uses it when available, falling
back to per-user fetching on older versions.
the UI now handles keys where user is null, showing them as
Tag Only in the filter dropdown.
closes#432
Amp-Thread-ID: https://ampcode.com/threads/T-019c9861-2d45-73ef-ab94-5fce62f63670
Co-authored-by: Amp <amp@ampcode.com>
Aarnav Tale