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>

home-manager/gnome-hm.nix

@@ -57,7 +57,7 @@
       
       "org/gnome/settings-daemon/plugins/media-keys/custom-keybindings/custom0" = {
         name = "Terminal";
-        command = "kgx";
+        command = "ghostty";
         binding = "<Super>t";
       };