fix(desktop): switch the terminal from Ghostty to Alacritty

Ghostty's GTK4 window construction made every launch sluggish on neogaia's
integrated graphics (~440 ms to map a window), which a head-to-head
comparison against Alacritty confirmed. Alacritty's lightweight OpenGL
renderer opens fast on the iGPU, so it becomes the terminal on Super+Return.

The choice stays reversible per host, so a capable host such as zeus could
adopt Ghostty later.

modules/desktop/hyprland/hyprland.nix

@@ -50,7 +50,7 @@ in
 
       settings = {
         "$mod" = "SUPER";
-        "$terminal" = "ghostty";
+        "$terminal" = "alacritty";
 
         input = {
           kb_layout = "us";

modules/desktop/terminal.nix

@@ -1,23 +1,19 @@
 {
   config,
   lib,
+  pkgs,
   ...
 }:
-# Ghostty as the desktop terminal.
+# Alacritty as the desktop terminal, a lightweight GPU renderer that opens
+# fast on the laptop's integrated graphics.
 let
   cfg = config.modules.desktop.terminal;
   user = config.user.name;
 in
 {
-  options.modules.desktop.terminal.enable = lib.mkEnableOption "Ghostty as the desktop terminal";
+  options.modules.desktop.terminal.enable = lib.mkEnableOption "Alacritty as the desktop terminal";
 
   config = lib.mkIf cfg.enable {
-    home-manager.users.${user}.programs.ghostty = {
-      enable = true;
-
-      # Reuse a single process, so each window after the first opens instantly
-      # instead of paying a fresh GTK startup.
-      settings.gtk-single-instance = true;
-    };
+    home-manager.users.${user}.home.packages = [ pkgs.alacritty ];
   };
 }