system: extract shared configuration from host configs

Extract duplicated configuration from gunter and magicman into shared
system modules. This eliminates ~116 lines of duplication and fixes a
malformed gdm.wayland config in magicman.

New shared modules:
- boot.nix: systemd-boot, EFI, common kernel params
- networking.nix: NetworkManager, nftables, firewall base
- hyprland.nix: System-level Hyprland and display manager
- xdg.nix: XDG session variables and portal setup
- nix-config.nix: Nix daemon settings, binary caches, trusted users
- nixpkgs-config.nix: allowUnfree and kernel packages default

Updated modules:
- locale.nix: Added timezone configuration

Benefits:
- Reduces duplication: net reduction of 53 lines
- Magicman gains custom cache substituters for faster builds
- Fixes malformed gdm.wayland config in magicman
- Both hosts use identical base configuration
- Host-specific config clearly stands out

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

system/default.nix

@@ -1,16 +1,22 @@
 {
   imports = [
+    ./boot.nix
     ./fonts.nix
-    ./root-ca.nix
     ./fwupd.nix
     ./git.nix
     ./greetd.nix
+    ./hyprland.nix
+    ./label.nix
     ./libvirt.nix
     ./locale.nix
+    ./networking.nix
+    ./nix-config.nix
+    ./nixpkgs-config.nix
     ./podman.nix
+    ./root-ca.nix
     ./security.nix
     ./services.nix
     ./users.nix
-    ./label.nix
+    ./xdg.nix
   ];
 }