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/default.nix

@@ -1,4 +1,9 @@
-{ pkgs, lib, config, ... }:
+{
+  pkgs,
+  lib,
+  config,
+  ...
+}:
 
 let
   # Deterministic Silicon Volt wallpaper: rendered from SVG at build time,
@@ -8,65 +13,65 @@ let
   '';
 in
 {
-    imports = [
-        ./hyprland.nix
-        ./hyprlock.nix
-        ./hypridle.nix
-        ./quickshell.nix
-    ];
+  imports = [
+    ./hyprland.nix
+    ./hyprlock.nix
+    ./hypridle.nix
+    ./quickshell.nix
+  ];
 
-    home.packages = with pkgs; [
-        # Utilities & Tools
-        grim
-        slurp
-        wl-clipboard
-        cliphist
-        brightnessctl
-        jq
+  home.packages = with pkgs; [
+    # Utilities & Tools
+    grim
+    slurp
+    wl-clipboard
+    cliphist
+    brightnessctl
+    jq
 
-        # Desktop Environment / Hyprland ecosystem
-        hyprpaper
-        hyprpolkitagent
-    ];
+    # Desktop Environment / Hyprland ecosystem
+    hyprpaper
+    hyprpolkitagent
+  ];
 
-    # Build-time rendered wallpaper
-    xdg.dataFile."backgrounds/silicon-volt.png".source = wallpaper;
+  # Build-time rendered wallpaper
+  xdg.dataFile."backgrounds/silicon-volt.png".source = wallpaper;
 
-    # hyprpaper: load the Silicon Volt wallpaper on every output
-    xdg.configFile."hypr/hyprpaper.conf".text = ''
-        preload = ${config.home.homeDirectory}/.local/share/backgrounds/silicon-volt.png
-        wallpaper = ,${config.home.homeDirectory}/.local/share/backgrounds/silicon-volt.png
-        splash = false
-    '';
+  # hyprpaper: load the Silicon Volt wallpaper on every output
+  xdg.configFile."hypr/hyprpaper.conf".text = ''
+    preload = ${config.home.homeDirectory}/.local/share/backgrounds/silicon-volt.png
+    wallpaper = ,${config.home.homeDirectory}/.local/share/backgrounds/silicon-volt.png
+    splash = false
+  '';
 
-    # Notifications: flat brutalist-glass cards with volt accents
-    services.mako = {
-        enable = true;
+  # Notifications: flat brutalist-glass cards with volt accents
+  services.mako = {
+    enable = true;
 
-        settings = {
-            background-color = "#1c1b1bee";
-            text-color = "#e5e2e1";
-            border-color = "#cffc00";
-            border-size = 1;
-            border-radius = 0;
-            width = 420;
-            margin = "12";
-            padding = "14";
-            default-timeout = 6000;
-            font = "JetBrainsMono Nerd Font 11";
-            format = "<b>%s</b>\\n%b";
-            "icon-location" = "left";
-            max-icon-size = 48;
+    settings = {
+      background-color = "#1c1b1bee";
+      text-color = "#e5e2e1";
+      border-color = "#cffc00";
+      border-size = 1;
+      border-radius = 0;
+      width = 420;
+      margin = "12";
+      padding = "14";
+      default-timeout = 6000;
+      font = "JetBrainsMono Nerd Font 11";
+      format = "<b>%s</b>\\n%b";
+      "icon-location" = "left";
+      max-icon-size = 48;
 
-            "urgency=critical" = {
-                border-color = "#ff7a59";
-                text-color = "#ffffff";
-                default-timeout = 0;
-            };
-            "urgency=low" = {
-                border-color = "#2a2929";
-                text-color = "#adaaaa";
-            };
-        };
+      "urgency=critical" = {
+        border-color = "#ff7a59";
+        text-color = "#ffffff";
+        default-timeout = 0;
+      };
+      "urgency=low" = {
+        border-color = "#2a2929";
+        text-color = "#adaaaa";
+      };
     };
+  };
 }