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/nix-config.nix

@@ -0,0 +1,32 @@
+{ ... }:
+{
+  nix.settings = {
+    experimental-features = [
+      "nix-command"
+      "flakes"
+    ];
+
+    trusted-users = [
+      "root"
+      "torjus"
+    ];
+
+    substituters = [
+      "https://nix-cache.home.2rjus.net"
+      "https://cache.nixos.org"
+      "https://cuda-maintainers.cachix.org"
+    ];
+
+    trusted-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="
+    ];
+  };
+}