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/boot.nix

@@ -0,0 +1,13 @@
+{ pkgs, lib, ... }:
+{
+  boot.loader.systemd-boot.enable = true;
+  boot.loader.efi.canTouchEfiVariables = true;
+
+  boot.kernelPackages = lib.mkDefault pkgs.linuxPackages_latest;
+
+  boot.kernelParams = lib.mkBefore [
+    "quiet"
+    "splash"
+    "rd.systemd.show_status=false"
+  ];
+}