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

hosts/hades/default.nix

@@ -11,18 +11,21 @@
     inputs.disko.nixosModules.disko
     ./disko.nix
     ./hardware-configuration.nix
-    ./display-configuration.nix
     ../../modules/hardware/nvidia.nix
-    ../../modules/apps/gaming.nix
-    ../../modules/apps/local_ai.nix
   ];
 
   networking.hostName = "hades";
 
+  # Opt this machine into the shared optional feature modules.
+  my = {
+    gaming.enable = true;
+    ai.local.enable = true;
+  };
+
   # ---------------------------------------------------------------------------
   # Host display configuration (absorbed by home/hypr/hyprland.nix)
   # ---------------------------------------------------------------------------
-  home-manager.users.anish.volt.monitors = [
+  home-manager.users.${config.my.username}.volt.monitors = [
     {
       # Samsung Odyssey Ark - matched by description so it survives port changes
       output = "desc:Samsung Electric Company Odyssey Ark H1AK500000";
@@ -44,7 +47,7 @@
 
   # Ensure /games directory is accessible by default user
   systemd.tmpfiles.rules = [
-    "d /games 0755 anish users -"
+    "d /games 0755 ${config.my.username} users -"
   ];
 
   powerManagement.cpuFreqGovernor = "performance";
@@ -58,7 +61,6 @@
   };
 
   environment.systemPackages = with pkgs; [
-    mangohud
     nvtopPackages.nvidia
   ];
 }