feat(sddm): seed last-user state declaratively

SDDM only prefills the username from /var/lib/sddm/state.conf, which
is written after a successful login — so the first login after a
display-manager switch shows an empty username field. Seed the file
from the host's khanelinix.user via a tmpfiles C rule (copy only when
missing), leaving SDDM's own last-user tracking in charge afterward.

Author: khaneliman

modules/nixos/display-managers/sddm/default.nix

@@ -14,6 +14,13 @@ let
   userName = config.khanelinix.user.name;
 
   themePackage = pkgs.catppuccin-sddm-corners;
+
+  # SDDM only prefills the username from state written after a successful
+  # login; seed it so the very first login is already populated.
+  seedStateFile = pkgs.writeText "sddm-state.conf" ''
+    [Last]
+    User=${userName}
+  '';
 in
 {
   options.khanelinix.display-managers.sddm = {
@@ -49,6 +56,12 @@ in
       };
     };
 
+    # C = copy only when the destination is missing, so SDDM's own last-user
+    # tracking still wins after the first login (C+ would force it every boot)
+    systemd.tmpfiles.rules = [
+      "C /var/lib/sddm/state.conf 0600 sddm sddm - ${seedStateFile}"
+    ];
+
     system.activationScripts.postInstallSddm = stringAfter [ "users" ] /* Bash */ ''
       echo "Setting sddm permissions for user icon"
       ${getExe' pkgs.acl "setfacl"} -m u:sddm:x /home/${userName}