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

flake.nix

@@ -4,7 +4,10 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
     nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
-    nixos-hardware.url = "github:NixOS/nixos-hardware/master";
+    nixos-hardware = {
+      url = "github:NixOS/nixos-hardware/master";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
 
     home-manager = {
       url = "github:nix-community/home-manager/release-26.05";
@@ -23,9 +26,9 @@
 
     comfy-ui = {
       url = "github:utensils/comfyui-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
     };
 
-
     herdr = {
       url = "github:herdrdev/herdr";
       inputs.nixpkgs.follows = "nixpkgs";
@@ -33,6 +36,7 @@
 
     pi = {
       url = "github:lukasl-dev/pi.nix";
+      inputs.nixpkgs.follows = "nixpkgs";
     };
   };
 
@@ -46,26 +50,18 @@
         ./modules/graphical
         ./modules/apps
         inputs.home-manager.nixosModules.home-manager
-        {
-          nixpkgs.overlays = [
-            (final: prev: {
-              unstable = import inputs.nixpkgs-unstable {
-                inherit (final) system;
-                config.allowUnfree = true;
-              };
-            })
-            inputs.antigravity.overlays.default
-            inputs.comfy-ui.overlays.default
-          ];
-
-          home-manager = {
-            useGlobalPkgs = true;
-            useUserPackages = true;
-            extraSpecialArgs = { inherit inputs; };
-            backupFileExtension = "backup";
-            users.anish = import ./home;
-          };
-        }
+        (
+          { config, ... }:
+          {
+            home-manager = {
+              useGlobalPkgs = true;
+              useUserPackages = true;
+              extraSpecialArgs = { inherit inputs; };
+              backupFileExtension = "backup";
+              users.${config.my.username} = import ./home;
+            };
+          }
+        )
       ];
     in
     {
@@ -80,6 +76,8 @@
         };
       };
 
+      formatter.${system} = inputs.nixpkgs.legacyPackages.${system}.nixfmt;
+
       devShells.${system}.default =
         let
           pkgs = import inputs.nixpkgs {