Add USB thumb drive install docs and double-click launcher

README: rewrite USB section with three steps:
  1. Auto-detect USB mount point and clone repo onto it
  2. Install nautilus-extension-gnome-terminal for right-click "Open in Terminal"
  3. Double-click "Run Setup.desktop" to launch wizard with sudo (no terminal needed)

Run Setup.desktop: launcher file included in the repo so it's present on the
USB automatically. Uses %k to find its own location, cds to that directory,
and runs sudo ./setup.sh in a terminal window.

https://claude.ai/code/session_017WJtGcE5jjerAQCUBWUE3H

Run Setup.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Version=1.0
+Type=Application
+Name=Ubuntu Post-Install Setup
+Comment=Run the post-install setup wizard with sudo
+Exec=bash -c 'cd "$(dirname "$1")" && sudo ./setup.sh; echo; read -rp "Done — press Enter to close" _' -- %k
+Terminal=true
+Icon=utilities-terminal
+Categories=System;