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/local-ai.nix

@@ -0,0 +1,45 @@
+{
+  config,
+  lib,
+  pkgs,
+  inputs,
+  ...
+}:
+
+{
+  imports = [
+    inputs.comfy-ui.nixosModules.default
+  ];
+
+  config = lib.mkIf config.my.ai.local.enable {
+    services = {
+      comfyui = {
+        enable = true;
+        gpuSupport = "cuda";
+        enableManager = true;
+        port = 8188;
+        listenAddress = "0.0.0.0";
+        dataDir = "/var/lib/comfyui";
+        user = "comfyui";
+        group = "comfyui";
+        createUser = true;
+      };
+
+      ollama = {
+        enable = true;
+        package = pkgs.unstable.ollama-cuda;
+        host = "0.0.0.0";
+        port = 11434;
+        user = "ollama";
+        group = "ollama";
+        home = "/var/lib/ollama";
+      };
+
+      open-webui.enable = true;
+    };
+
+    environment.systemPackages = with pkgs; [
+      unstable.ollama-cuda
+    ];
+  };
+}