fix broken references to deleted files, remove dead code

Build-breaking fixes:
- kibbeh: remove imports of deleted admin-essentials, nix, zsh configs
  (all provided by niphas via profiles.default + profiles.desktop)
- tabula, tahina: remove imports of deleted admin-essentials, nix configs
  (provided by niphas via profiles.default)
- zaatar: remove import of deleted tmux config
  (provided by niphas via profiles.default)

Dead code removal:
- flake.nix: remove nixosModules.zsh-kmein (referenced deleted config/zsh.nix)
- flake.nix: remove unused wallpapers input
- packages/gpt.nix, hora.nix, k-lock.nix: orphaned, not in overlay or referenced
- configs/hledger.nix: remove hora reference (package deleted)

systems/kibbeh/configuration.nix

@@ -8,17 +8,14 @@
   imports = [
     ./hardware-configuration.nix
     ../../configs/spacetime.nix
-    ../../configs/admin-essentials.nix
     ../../configs/keyboard
     ../../configs/sound.nix
     ../../configs/printing.nix
-    ../../configs/nix.nix
     ../../configs/fonts.nix
     ../../configs/mycelium.nix
     ../../configs/retiolum.nix
     ../../configs/sshd.nix
     ../../configs/sudo.nix
-    ../../configs/zsh.nix
   ];
 
   age.secrets = {

systems/tabula/configuration.nix

@@ -10,8 +10,6 @@
     ../../configs/spacetime.nix
     ../../configs/retiolum.nix
     ../../configs/sshd.nix
-    ../../configs/nix.nix
-    ../../configs/admin-essentials.nix
   ];
 
   age.secrets = {

systems/tahina/configuration.nix

@@ -10,8 +10,6 @@
     ../../configs/spacetime.nix
     ../../configs/sshd.nix
     ../../configs/retiolum.nix
-    ../../configs/nix.nix
-    ../../configs/admin-essentials.nix
   ];
 
   age.secrets = {

systems/zaatar/configuration.nix

@@ -11,7 +11,6 @@
     ./hardware-configuration.nix
     ./home-assistant.nix
     ../../configs/printing.nix
-    ../../configs/tmux.nix
     ../../configs/wpa_supplicant.nix
   ];