fix(ipa): restore IPA password login via LightDM and su

Two independent fixes:

1. hashedPassword = "!" on the IPA user stub
   Without any shadow entry, pam_unix returns PAM_AUTHINFO_UNAVAIL
   before prompting, so PAM_AUTHTOK is never set. The "!" marker
   (account locked, not a real hash) ensures a shadow entry exists so
   pam_unix prompts and sets PAM_AUTHTOK — which the subsequent
   pam_sss module can then use.

2. pam_sss try_first_pass instead of use_first_pass (login + su)
   use_first_pass silently fails when PAM_AUTHTOK is unset.
   try_first_pass prompts independently in that case, making IPA
   password auth work even if pam_unix returns early for any reason.

Root cause found during incident: wayne (uid 50002) was outside IPA's
auto-SID range (163800000+), so sidgen never assigned him an
ipaNTSecurityIdentifier. Without it, ipadb's handle_authdata step fails
with "Generic error" after SPAKE pre-auth succeeds. Fixed by manually
adding objectClass: ipaNTUserAttrs + ipaNTSecurityIdentifier RID 550002
to wayne's LDAP entry on domain-controller. Any future IPA user with a
manually-assigned uid outside the auto-range needs the same treatment.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/ipa/client.nix

@@ -60,6 +60,15 @@ lib.mkIf enabled {
     pam.services = {
       sshd.makeHomeDir = true;
       lightdm.makeHomeDir = true;
+
+      # pam_unix returns PAM_AUTHINFO_UNAVAIL without prompting when the local
+      # stub has "!" in shadow (account locked), so PAM_AUTHTOK is never set
+      # and pam_sss's use_first_pass fails with "No authentication token".
+      # Changing to try_first_pass makes pam_sss prompt independently when no
+      # prior module has set the token, restoring IPA password login via
+      # LightDM and su.
+      login.rules.auth.sss.settings = lib.mkForce { try_first_pass = true; };
+      su.rules.auth.sss.settings = lib.mkForce { try_first_pass = true; };
     };
 
     # HM with useUserPackages = true (flake.nix) sets users.users.${ipaUser}.packages,
@@ -170,6 +179,13 @@ lib.mkIf enabled {
     group = "users";
     extraGroups = [ "wheel" ];
     createHome = false;
+    # "!" is not a password hash — it is the standard "account locked" marker.
+    # It cannot authenticate anyone locally. It exists solely so NixOS generates
+    # a shadow entry for this stub user; without one pam_unix returns
+    # PAM_AUTHINFO_UNAVAIL before prompting, which means PAM_AUTHTOK is never
+    # set and the subsequent pam_sss use_first_pass call has nothing to work
+    # with — blocking LightDM and su logins even when IPA/SSSD auth succeeds.
+    hashedPassword = "!";
   };
 
   # Home Manager config for the IPA primary user, applied on every enrolled