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>

hosts/gunter/configuration.nix

@@ -31,14 +31,8 @@
       "mt76"
     ];
     # Kernel stuff
-    kernelPackages = pkgs.linuxPackages_latest;
     # kernelPackages = lib.warn "Pinned to kernel 6.12 due to issues" pkgs.linuxPackages_6_12;
-    kernelParams = [
-      "quiet"
-      "splash"
-      "rd.systemd.show_status=false"
-      "module_blacklist=amdgpu"
-    ];
+    kernelParams = [ "module_blacklist=amdgpu" ];
 
     kernel.sysctl = {
       "vm.max_map_count" = 262144;
@@ -50,30 +44,34 @@
 
     # Bootloader stuff
     loader.systemd-boot = {
-      enable = true;
       configurationLimit = 10;
       memtest86.enable = true;
     };
-    loader.efi = {
-      canTouchEfiVariables = true;
-    };
     supportedFilesystems = [ "nfs" ];
   };
 
   # Networking stuff
   networking.hostName = "gunter"; # Define your hostname.
-  networking.networkmanager.enable = true;
-  networking.nftables.enable = true;
-  networking.firewall = {
-    enable = true;
-    allowedTCPPorts = [
-      8989
+  networking.firewall.allowedTCPPorts = [ 8989 ];
+
+  # Additional nix caches for homelab and CUDA
+  nix.settings = {
+    substituters = [
+      "https://nix-cache.home.2rjus.net"
+      "https://cuda-maintainers.cachix.org"
+    ];
+
+    trusted-substituters = [
+      "https://nix-cache.home.2rjus.net"
+      "https://cuda-maintainers.cachix.org"
+    ];
+
+    trusted-public-keys = [
+      "nix-cache.home.2rjus.net-1:2kowZOG6pvhoK4AHVO3alBlvcghH20wchzoR0V86UWI="
+      "cuda-maintainers.cachix.org-1:0dq3bujKpuEPMCX6U4WylrUDZ9JyUG0VpVZa7CNfq5E="
     ];
   };
 
-  # Set time stuff
-  time.timeZone = "Europe/Oslo";
-
   # Enable graphics
   hardware.graphics = {
     enable = true;
@@ -105,7 +103,7 @@
     #     };
   };
 
-  # Setup hyprland
+  # Setup nvidia video drivers
   # nixpkgs.overlays = [
   #   (self: super: {
   #     hyprland = super.hyprland.override {
@@ -113,68 +111,8 @@
   #     };
   #   })
   # ];
-  services.displayManager.gdm.wayland = true;
-
-  services.xserver.enable = true;
   services.xserver.videoDrivers = [ "nvidia" ];
-  programs.hyprland = {
-    enable = true;
-    withUWSM = true;
-    xwayland.enable = true;
-    portalPackage = pkgs.xdg-desktop-portal-hyprland;
-  };
 
-  # Setup common XDG env vars
-  environment.sessionVariables = rec {
-    XDG_CACHE_HOME = "$HOME/.cache";
-    XDG_CONFIG_HOME = "$HOME/.config";
-    XDG_DATA_HOME = "$HOME/.local/share";
-    XDG_STATE_HOME = "$HOME/.local/state";
-    XDG_BIN_HOME = "$HOME/.local/bin";
-    PATH = [ "${XDG_BIN_HOME}" ];
-  };
-
-  # Setup xdg portal
-  xdg.portal = {
-    enable = true;
-    xdgOpenUsePortal = true;
-    extraPortals = (
-      with pkgs;
-      [
-        # unstable.xdg-desktop-portal-hyprland
-        xdg-desktop-portal-gtk
-      ]
-    );
-  };
-
-  # Enable flakes
-  nix.settings = {
-    experimental-features = [
-      "nix-command"
-      "flakes"
-    ];
-    trusted-users = [
-      "root"
-      "torjus"
-    ];
-    trusted-substituters = [
-      "https://nix-cache.home.2rjus.net"
-      "https://cache.nixos.org"
-      "https://cuda-maintainers.cachix.org"
-    ];
-    substituters = [
-      "https://nix-cache.home.2rjus.net"
-      "https://cache.nixos.org"
-      "https://cuda-maintainers.cachix.org"
-    ];
-    trusted-public-keys = [
-      "nix-cache.home.2rjus.net-1:2kowZOG6pvhoK4AHVO3alBlvcghH20wchzoR0V86UWI="
-      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
-      "cuda-maintainers.cachix.org-1:0dq3bujKpuEPMCX6U4WylrUDZ9JyUG0VpVZa7CNfq5E="
-    ];
-  };
-
-  nixpkgs.config.allowUnfree = true;
   # Install system-wide packages
   environment.systemPackages = with pkgs; [
     curl