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>

common.nix

@@ -69,7 +69,11 @@
   networking.networkmanager.enable = true;
   networking.nameservers = [ "1.1.1.1" "9.9.9.9" ];
 
-  # Shell aliases
+  # Fish shell
+  programs.fish.enable = true;
+  users.defaultUserShell = pkgs.fish;
+
+  # Shell aliases (work in both bash and fish)
   environment.shellAliases = {
     update = "sudo nixos-rebuild switch --flake github:ediblerope/nixos-config";
     clean = "sudo nix-collect-garbage -d";