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/default.nix

@@ -11,18 +11,21 @@
     inputs.disko.nixosModules.disko
     ./disko.nix
     ./hardware-configuration.nix
-    ./display-configuration.nix
     ../../modules/hardware/nvidia.nix
-    ../../modules/apps/gaming.nix
-    ../../modules/apps/local_ai.nix
   ];
 
   networking.hostName = "hades";
 
+  # Opt this machine into the shared optional feature modules.
+  my = {
+    gaming.enable = true;
+    ai.local.enable = true;
+  };
+
   # ---------------------------------------------------------------------------
   # Host display configuration (absorbed by home/hypr/hyprland.nix)
   # ---------------------------------------------------------------------------
-  home-manager.users.anish.volt.monitors = [
+  home-manager.users.${config.my.username}.volt.monitors = [
     {
       # Samsung Odyssey Ark - matched by description so it survives port changes
       output = "desc:Samsung Electric Company Odyssey Ark H1AK500000";
@@ -44,7 +47,7 @@
 
   # Ensure /games directory is accessible by default user
   systemd.tmpfiles.rules = [
-    "d /games 0755 anish users -"
+    "d /games 0755 ${config.my.username} users -"
   ];
 
   powerManagement.cpuFreqGovernor = "performance";
@@ -58,7 +61,6 @@
   };
 
   environment.systemPackages = with pkgs; [
-    mangohud
     nvtopPackages.nvidia
   ];
 }

hosts/hades/disko.nix

@@ -14,7 +14,10 @@
                 type = "filesystem";
                 format = "vfat";
                 mountpoint = "/boot";
-                mountOptions = [ "fmask=0077" "dmask=0077" ];
+                mountOptions = [
+                  "fmask=0077"
+                  "dmask=0077"
+                ];
               };
             };
             root = {

hosts/hades/display-configuration.nix

@@ -1,5 +0,0 @@
-{ ... }:
-
-{
-  # Host-specific display configurations can be placed here if needed.
-}

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 = [ ];