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/networking.nix

@@ -0,0 +1,8 @@
+{ lib, ... }:
+{
+  networking.networkmanager.enable = true;
+  networking.nftables.enable = true;
+  networking.firewall.enable = true;
+  networking.firewall.allowedTCPPorts = lib.mkDefault [ ];
+  networking.firewall.allowedUDPPorts = lib.mkDefault [ ];
+}