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

home/brave.nix

@@ -29,25 +29,26 @@ let
   };
 in
 {
-    programs.chromium = {
-        enable = true;
-        package = pkgs.brave;
-        
-        commandLineArgs = [
-            "--enable-features=UseOzonePlatform"
-            "--ozone-platform=wayland"
-            "--enable-wayland-ime"
-            "--ignore-gpu-blocklist"
-            "--enable-gpu-rasterization"
-            "--enable-zero-copy"
-        ];
-        
-        extensions = [
-            { id = "nngceckbapebfimnlniiiahkandclblb"; } # Bitwarden
-            { id = "hfjbmagddngcpeloejdejnfgbamkjaeg"; } # Vimium C
-        ]; 
-    };
-    xdg.configFile."brave/policies/managed/policies.json".text = builtins.toJSON bravePolicies;
-    xdg.configFile."BraveSoftware/Brave-Browser/policies/managed/policies.json".text = builtins.toJSON bravePolicies;
-    xdg.configFile."chromium/policies/managed/policies.json".text = builtins.toJSON bravePolicies;
+  programs.chromium = {
+    enable = true;
+    package = pkgs.brave;
+
+    commandLineArgs = [
+      "--enable-features=UseOzonePlatform"
+      "--ozone-platform=wayland"
+      "--enable-wayland-ime"
+      "--ignore-gpu-blocklist"
+      "--enable-gpu-rasterization"
+      "--enable-zero-copy"
+    ];
+
+    extensions = [
+      { id = "nngceckbapebfimnlniiiahkandclblb"; } # Bitwarden
+      { id = "hfjbmagddngcpeloejdejnfgbamkjaeg"; } # Vimium C
+    ];
+  };
+  xdg.configFile."brave/policies/managed/policies.json".text = builtins.toJSON bravePolicies;
+  xdg.configFile."BraveSoftware/Brave-Browser/policies/managed/policies.json".text =
+    builtins.toJSON bravePolicies;
+  xdg.configFile."chromium/policies/managed/policies.json".text = builtins.toJSON bravePolicies;
 }