fix(install): resolve versioned python3-venv package; skip XFCE-only apt packages off XFCE

Newer distros drop the python3-venv transitional package once it no longer
resolves to a real pythonX.Y-venv candidate, which crashed sync.py's venv
creation with a raw traceback. Prep now resolves the exact versioned
package via apt-cache, and skips xfce4-genmon-plugin/plank/blueman on
boxes with no xfconf-query so a non-XFCE box doesn't get noisy "unable to
locate package" failures for a panel it'll never run. venv_python() also
fails with an actionable message instead of an uncaught CalledProcessError.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

bin/sync.py

@@ -62,7 +62,14 @@ def venv_python() -> Path:
     venv = ROOT / "Promethean"
     py = venv / ("Scripts/python.exe" if os.name == "nt" else "bin/python")
     if not py.exists():
-        subprocess.run([sys.executable, "-m", "venv", str(venv)], check=True)
+        result = subprocess.run([sys.executable, "-m", "venv", str(venv)])
+        if result.returncode:
+            raise SystemExit(
+                "\nvenv creation failed - the Python 'venv' module isn't usable here.\n"
+                "On Debian/Ubuntu this is usually a missing pythonX.Y-venv package; "
+                "check the 'Installing system packages' warning further up, install "
+                "it by hand, then re-run."
+            )
     return py