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/virt.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ config, pkgs, ... }:
 
 {
   virtualisation = {
@@ -27,7 +27,7 @@
 
   programs.virt-manager.enable = true;
 
-  users.users.anish.extraGroups = [
+  users.users.${config.my.username}.extraGroups = [
     "libvirtd"
     "kvm"
     "incus-admin"