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/hypr/quickshell.nix

@@ -1,9 +1,17 @@
 { pkgs, inputs, ... }:
 
 let
-  quickshellPkg = if (inputs ? quickshell && inputs.quickshell ? packages && inputs.quickshell.packages ? ${pkgs.stdenv.hostPlatform.system})
-    then inputs.quickshell.packages.${pkgs.stdenv.hostPlatform.system}.default
-    else (pkgs.quickshell or pkgs.qt6.qtbase);
+  quickshellPkg =
+    if
+      (
+        inputs ? quickshell
+        && inputs.quickshell ? packages
+        && inputs.quickshell.packages ? ${pkgs.stdenv.hostPlatform.system}
+      )
+    then
+      inputs.quickshell.packages.${pkgs.stdenv.hostPlatform.system}.default
+    else
+      (pkgs.quickshell or pkgs.qt6.qtbase);
 in
 {
   home.packages = with pkgs; [