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/options.nix

@@ -0,0 +1,20 @@
+{
+  lib,
+  ...
+}:
+
+{
+  # Shared, machine-agnostic configuration surface. Hosts enable optional
+  # feature modules by setting these booleans instead of editing import lists.
+  options.my = {
+    username = lib.mkOption {
+      type = lib.types.str;
+      default = "anish";
+      description = "Primary interactive user account.";
+    };
+
+    gaming.enable = lib.mkEnableOption "Steam, Gamemode and gaming tooling";
+
+    ai.local.enable = lib.mkEnableOption "local AI services (Ollama, ComfyUI, Open WebUI)";
+  };
+}