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/browser.nix

@@ -1,5 +0,0 @@
-{ ... }:
-
-{
-    # Browser is configured and managed via Home Manager (home/brave.nix)
-}

modules/apps/communication.nix

@@ -1,8 +1,8 @@
 { pkgs, ... }:
 
 {
-    environment.systemPackages = with pkgs; [
-        vesktop
-        signal-desktop
-    ];
+  environment.systemPackages = with pkgs; [
+    vesktop
+    signal-desktop
+  ];
 }

modules/apps/default.nix

@@ -3,9 +3,10 @@
 {
   imports = [
     ./terminal.nix
-    ./browser.nix
     ./communication.nix
     ./dev.nix
     ./virt.nix
+    ./gaming.nix
+    ./local-ai.nix
   ];
 }

modules/apps/gaming.nix

@@ -1,15 +1,22 @@
-{ pkgs, ... }:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
 
 {
+  config = lib.mkIf config.my.gaming.enable {
     programs.steam = {
-        enable = true;
-        remotePlay.openFirewall = false;
-        dedicatedServer.openFirewall = false;
-        gamescopeSession.enable = false;
+      enable = true;
+      remotePlay.openFirewall = false;
+      dedicatedServer.openFirewall = false;
+      gamescopeSession.enable = false;
     };
 
     environment.systemPackages = with pkgs; [
-        mangohud
-        gamemode
+      mangohud
+      gamemode
     ];
+  };
 }

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
+    ];
+  };
+}

modules/apps/local_ai.nix

@@ -1,37 +0,0 @@
-{ pkgs, inputs, ... }:
-
-{
-  imports = [
-    inputs.comfy-ui.nixosModules.default
-  ];
-
-  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
-  ];
-}

modules/apps/terminal.nix

@@ -1,9 +1,9 @@
 { pkgs, ... }:
 
 {
-    # Terminal is managed elsewhere; system-level CLI helpers live here.
-    environment.systemPackages = with pkgs; [
-        zoxide
-        fzf
-    ];
+  # Terminal is managed elsewhere; system-level CLI helpers live here.
+  environment.systemPackages = with pkgs; [
+    zoxide
+    fzf
+  ];
 }

modules/apps/virt.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ config, pkgs, ... }:
 
 {
   virtualisation = {
@@ -27,7 +27,7 @@
 
   programs.virt-manager.enable = true;
 
-  users.users.anish.extraGroups = [
+  users.users.${config.my.username}.extraGroups = [
     "libvirtd"
     "kvm"
     "incus-admin"