fix(tailscale): open Tailscale's WireGuard port in the firewall

services.tailscale.openFirewall defaults to false while the NixOS
firewall is enabled, so argon, krypton and neon dropped unsolicited
inbound 41641/udp. Direct connections then only succeed if the other
peer accepts inbound; between two of these hosts (or a host and a
phone) traffic can fall back to a DERP relay.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

modules/nixos/services.nix

@@ -4,6 +4,10 @@
     tuned.enable = true;
     upower.enable = true;
     fwupd.enable = true;
-    tailscale.enable = true;
+    tailscale = {
+      enable = true;
+      # inbound 41641/udp so peers can connect directly instead of via DERP
+      openFirewall = true;
+    };
   };
 }