Add GUI git-askpass so credential prompts work without a TTY

Installs /usr/local/bin/git-askpass (uses zenity or qarma) and sets
core.askPass in /etc/gitconfig. Git now pops a GUI dialog for username/
password instead of trying to open /dev/tty, which fails in headless
contexts (scripts, Claude Code terminal).

Adds zenity to live desktop package lists for both niri and cinnamon profiles.
Propagated to: both live ISO builders and the installer postinstall.

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

config/profiles/mainline-niri/packages.live-desktop.list

@@ -91,6 +91,7 @@ python3
 upower
 power-profiles-daemon
 wl-clipboard
+zenity
 
 # --- XDG portals ---
 xdg-desktop-portal

config/profiles/stable-cinnamon/packages.live-desktop.list

@@ -15,6 +15,7 @@ sudo
 bash
 bash-completion
 git
+zenity
 curl
 wget
 vim