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/users.nix

@@ -1,30 +1,30 @@
-{ pkgs, ... }:
+{ config, pkgs, ... }:
 
 {
-    users.defaultUserShell = pkgs.zsh;
-    programs.zsh = {
-        enable = true;
-        loginShellInit = ''
-            if [ -z "$DISPLAY" ] && [ "$TTY" = "/dev/tty1" ]; then
-                exec start-hyprland
-            fi
-        '';
-    }; 
+  users.defaultUserShell = pkgs.zsh;
+  programs.zsh.enable = true;
 
-    users.users.anish = {
-        isNormalUser = true;
-        description = "Main Account";
-        extraGroups = [ "wheel" "networkmanager" "video" "audio" "gaming" "llm" ];
-        openssh.authorizedKeys.keys = [
-            # ssh-ed25519 AAAAA.... add my key here to enable ssh from another computer
-        ]; 
-    };
-    
-    services.openssh = {
-        enable = true;
-        settings = {
-            PasswordAuthentication = false;
-            PermitRootLogin = "no";
-        };
+  users.users.${config.my.username} = {
+    isNormalUser = true;
+    description = "Main Account";
+    extraGroups = [
+      "wheel"
+      "networkmanager"
+      "video"
+      "audio"
+      "gaming"
+      "llm"
+    ];
+    openssh.authorizedKeys.keys = [
+      # ssh-ed25519 AAAAA.... add my key here to enable ssh from another computer
+    ];
+  };
+
+  services.openssh = {
+    enable = true;
+    settings = {
+      PasswordAuthentication = false;
+      PermitRootLogin = "no";
     };
+  };
 }