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/hardware/nvidia.nix

@@ -1,34 +1,39 @@
-{ config, pkgs, lib, ... }:
+{
+  config,
+  pkgs,
+  lib,
+  ...
+}:
 
 {
-    hardware.graphics = {
-        enable = true;
-        enable32Bit = true;
-    };
+  services.xserver.videoDrivers = [ "nvidia" ];
 
-    services.xserver.videoDrivers = [ "nvidia" ];
+  boot.initrd.kernelModules = [
+    "nvidia"
+    "nvidia_modeset"
+    "nvidia_uvm"
+    "nvidia_drm"
+  ];
+  boot.kernelParams = [
+    "nvidia-drm.modeset=1"
+    "nvidia-drm.fbdev=1"
+  ];
 
-    boot.initrd.kernelModules = [ "nvidia" "nvidia_modeset" "nvidia_uvm" "nvidia_drm" ];
-    boot.kernelParams = [
-        "nvidia-drm.modeset=1"
-        "nvidia-drm.fbdev=1"
-    ];
-    
-    hardware.nvidia = {
-        modesetting.enable = true;
-        powerManagement.enable = true;
-        powerManagement.finegrained = lib.mkDefault false;
-        open = lib.mkDefault true;
-        nvidiaSettings = true;
-        package = config.boot.kernelPackages.nvidiaPackages.stable;
-    };
+  hardware.nvidia = {
+    modesetting.enable = true;
+    powerManagement.enable = true;
+    powerManagement.finegrained = lib.mkDefault false;
+    open = lib.mkDefault true;
+    nvidiaSettings = true;
+    package = config.boot.kernelPackages.nvidiaPackages.stable;
+  };
 
-    # NVIDIA-specific session variables live here so machines without
-    # NVIDIA GPUs (which do not import this module) stay vendor-clean.
-    environment.sessionVariables = {
-        LIBVA_DRIVER_NAME = "nvidia";
-        GBM_BACKEND = "nvidia-drm";
-        __GLX_VENDOR_LIBRARY_NAME = "nvidia";
-        NVD_BACKEND = "direct";
-    };
+  # NVIDIA-specific session variables live here so machines without
+  # NVIDIA GPUs (which do not import this module) stay vendor-clean.
+  environment.sessionVariables = {
+    LIBVA_DRIVER_NAME = "nvidia";
+    GBM_BACKEND = "nvidia-drm";
+    __GLX_VENDOR_LIBRARY_NAME = "nvidia";
+    NVD_BACKEND = "direct";
+  };
 }