Remove gnome-shell-extension-{caffeine,dashtodock,freon} from apt
and install all four extensions via gext (gnome-extensions-cli),
which pulls from extensions.gnome.org and avoids the version lag
of distro packages.

Extract a _gext_install helper to avoid repeating the
install+enable idempotency checks for each extension UUID.
This also fixes re-runs always calling gext install/enable even
when the extension was already present.

Extensions managed via gext:
- caffeine@patapon.info
- dash-to-dock@micxgx.gmail.com
- freon@UshakovVasilii_Github.yahoo.com
- clipboard-history@alexsaveau.dev

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>