Switch to fish shell, ghostty terminal, and simplified prompt

- Replace bash with fish as default shell (all hosts)
- Replace kgx with ghostty (desktop hosts), update Super+T keybinding
- Custom two-line fish prompt: NixOS icon, username, path, hostname, ❯
- Nix-shell awareness, red ❯ on error
- Simplify fastfetch: user@host, OS, kernel, shell, terminal, uptime, memory
- Ghostty config: FiraCode Nerd Font, catppuccin-mocha, no titlebar

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

settings/gnome.nix

@@ -8,8 +8,9 @@
     services.displayManager.gdm.wayland = true;
     boot.plymouth.enable = false;
     
-    # Add extensions and packages
+    # Add extensions, packages, and terminal
     environment.systemPackages = with pkgs; [
+      ghostty
       gnomeExtensions.blur-my-shell
       gnomeExtensions.just-perfection
       gnomeExtensions.appindicator