refactor: make hosts declarative and consolidate shared config

- add my.username / my.gaming.enable / my.ai.local.enable options and
  gate optional app modules on them instead of per-host import lists
- move vendor-neutral hardware.graphics into modules/graphical, leaving
  only NVIDIA-specific drivers and env vars in modules/hardware/nvidia
- derive the primary user everywhere from my.username (users, getty,
  home-manager, tmpfiles)
- de-duplicate Hyprland autostart down to a single system loginShellInit
  using the start-hyprland wrapper
- consolidate all nixpkgs overlays into modules/core/nix.nix and use
  final.stdenv.hostPlatform.system
- make the zsh rebuild helper target $(hostname) instead of hardcoded hades
- add nixpkgs follows for nixos-hardware, pi and comfy-ui (single nixpkgs)
- add .gitignore and a nixfmt flake formatter; format all nix files
- remove empty browser.nix/display-configuration.nix and rename
  local_ai.nix -> local-ai.nix
- document the my.* toggles in the README

modules/apps/gaming.nix

@@ -1,15 +1,22 @@
-{ pkgs, ... }:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
 
 {
+  config = lib.mkIf config.my.gaming.enable {
     programs.steam = {
-        enable = true;
-        remotePlay.openFirewall = false;
-        dedicatedServer.openFirewall = false;
-        gamescopeSession.enable = false;
+      enable = true;
+      remotePlay.openFirewall = false;
+      dedicatedServer.openFirewall = false;
+      gamescopeSession.enable = false;
     };
 
     environment.systemPackages = with pkgs; [
-        mangohud
-        gamemode
+      mangohud
+      gamemode
     ];
+  };
 }