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

hosts/hades/hardware-configuration.nix

@@ -1,14 +1,27 @@
 # Do not modify this file!  It was generated by ‘nixos-generate-config’
 # and may be overwritten by future invocations.  Please make changes
 # to /etc/nixos/configuration.nix instead.
-{ config, lib, pkgs, modulesPath, ... }:
+{
+  config,
+  lib,
+  pkgs,
+  modulesPath,
+  ...
+}:
 
 {
   imports = [
-      (modulesPath + "/installer/scan/not-detected.nix")
-    ];
+    (modulesPath + "/installer/scan/not-detected.nix")
+  ];
 
-  boot.initrd.availableKernelModules = [ "nvme" "xhci_pci" "ahci" "thunderbolt" "usbhid" "sd_mod" ];
+  boot.initrd.availableKernelModules = [
+    "nvme"
+    "xhci_pci"
+    "ahci"
+    "thunderbolt"
+    "usbhid"
+    "sd_mod"
+  ];
   boot.initrd.kernelModules = [ ];
   boot.kernelModules = [ "kvm-amd" ];
   boot.extraModulePackages = [ ];