Do not hold boot hostage for the tailnet join

Review caught that the join unit was Type=oneshot wanted by
multi-user.target: target units implicitly gain After= for their Wants=,
so an offline machine's boot -- SDDM included -- stalled for the full
10-minute timeout, and once timed out the join never retried within that
boot. Type=simple counts as started the moment it forks, so boot proceeds
while the join retries in the background for as long as the boot lasts.
Cleanup moves inside the script, sequenced after tailscale up succeeds,
which preserves the retry-across-boots behavior: key and unit survive a
boot with no connectivity, and the key never outlives a successful join.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xc74h4WSYpuYPGuYbFAR1q

README.md

@@ -52,7 +52,7 @@ ssh-ed25519 AAAA... you@host
 
 When `authorized_keys` is present, autoinstall installs it as the user's `~/.ssh/authorized_keys`, enables `sshd`, and adds a `ufw allow ssh` rule — a stock Omarchy install ships openssh with the service disabled and its firewall opens neither port 22 nor anything else beyond LocalSend. Networking needs nothing extra; NetworkManager is already enabled with DHCP. Password SSH authentication is left at the distro default. An `authorized_keys` with no usable keys fails the install rather than producing a machine nobody can reach.
 
-When `tailscale_authkey` is present (one key, blank lines and `#` comments ignored), the install adds the `tailscale` package from the ISO's bundled mirror — nothing is fetched from the network at install or boot — and stages the join for first boot: the key lands at `/etc/tailscale/authkey` (root-only), `tailscaled` is enabled, ufw allows traffic in on `tailscale0`, and a oneshot unit runs `tailscale up` once the network is actually up, retrying until it succeeds. After a successful join the key is deleted and the unit disables itself; until then both survive reboots, so a machine installed offline joins whenever it first gets connectivity. The node appears on the tailnet under the configured hostname. Use a reusable, pre-authorized (tagged) key so one drive image serves many machines — or an ephemeral key for disposable VMs.
+When `tailscale_authkey` is present (one key, blank lines and `#` comments ignored), the install adds the `tailscale` package from the ISO's bundled mirror — nothing is fetched from the network at install or boot — and stages the join for first boot: the key lands at `/etc/tailscale/authkey` (root-only), `tailscaled` is enabled, ufw allows traffic in on `tailscale0`, and a background unit runs `tailscale up` once the network is actually up, retrying until it succeeds without holding up the boot. After a successful join the key is deleted and the unit disables itself; until then both survive reboots, so a machine installed offline joins whenever it first gets connectivity. The node appears on the tailnet under the configured hostname. Use a reusable, pre-authorized (tagged) key so one drive image serves many machines — or an ephemeral key for disposable VMs.
 
 ### Building the drive