* contrib: desktop entry launches the GUI by absolute path

Desktop sessions (e.g. KDE launched from a display manager) often lack
~/.local/bin on PATH, so the relative Exec=signaltty-gui failed with
"could not find the program". The installer now writes the installed
binary's quoted absolute path into the entry and refuses prefixes whose
characters would need Desktop Entry escaping.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* contrib: reject relative desktop install prefixes and embedded control characters

* contrib: cover absolute desktop launcher installation

---------

Co-authored-by: Simone <185146821+Lucenx9@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>