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

home/rofi.nix

@@ -3,8 +3,7 @@
 let
   # Rofi script-mode indexer: exposes every executable user script
   # (shell, python, whatever) so the launcher finds "everything".
-  rofiScripts = pkgs.writeShellScriptBin "rofi-scripts"
-    (builtins.readFile ./rofi/rofi-scripts.sh);
+  rofiScripts = pkgs.writeShellScriptBin "rofi-scripts" (builtins.readFile ./rofi/rofi-scripts.sh);
 in
 {
   programs.rofi = {