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/core/nix.nix

@@ -36,12 +36,16 @@
 
   nixpkgs.config.allowUnfree = true;
 
+  # Single source of truth for overlays; the flake module imports only the
+  # modules and does not declare overlays itself.
   nixpkgs.overlays = [
     (final: prev: {
       unstable = import inputs.nixpkgs-unstable {
-        inherit (prev) system;
-        inherit (prev) config;
+        system = final.stdenv.hostPlatform.system;
+        config = final.config;
       };
     })
+    inputs.antigravity.overlays.default
+    inputs.comfy-ui.overlays.default
   ];
 }