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/xdg.nix

@@ -0,0 +1,17 @@
+{ pkgs, ... }:
+{
+  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}" ];
+  };
+
+  xdg.portal = {
+    enable = true;
+    xdgOpenUsePortal = true;
+    extraPortals = with pkgs; [ xdg-desktop-portal-gtk ];
+  };
+}