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/restore-linux.sh

@@ -33,9 +33,29 @@ if [ "$stage" = "prep" ]; then
     # installed, so it's cheap on an in-place restore. Skipped without apt (non-Debian).
     if command -v apt-get >/dev/null; then
         echo "Installing system packages..."
-        sudo apt-get install -y --no-install-recommends \
-            git sqlite3 curl python3-venv python3-gi gir1.2-gtk-3.0 \
-            xfce4-genmon-plugin plank blueman \
+
+        # python3-venv is a transitional package pointing at the real
+        # pythonX.Y-venv for whatever Python ships by default; on a distro ahead
+        # of that transition it resolves to nothing ("has no installation
+        # candidate") even though a real, version-suffixed package exists.
+        # apt-cache show only succeeds for a package with an actual archive
+        # entry, so it tells the two cases apart without guessing.
+        venv_pkg="python3-venv"
+        if command -v python3 >/dev/null && ! apt-cache show python3-venv >/dev/null 2>&1; then
+            venv_pkg="python3-$(python3 -c 'import sys; print(f"{sys.version_info.major}.{sys.version_info.minor}")')-venv"
+        fi
+
+        pkgs="git sqlite3 curl $venv_pkg python3-gi gir1.2-gtk-3.0"
+        # XFCE-only cosmetics: only worth attempting where there's an XFCE
+        # session to plug into. Same signal the desktop stage below already
+        # gates on, so a headless/WSL box (no xfconf-query) isn't left with
+        # noisy "unable to locate package" failures for a panel it'll never run.
+        if command -v xfconf-query >/dev/null; then
+            pkgs="$pkgs xfce4-genmon-plugin plank blueman"
+        fi
+
+        # shellcheck disable=SC2086  # $pkgs is a deliberate word-split list
+        sudo apt-get install -y --no-install-recommends $pkgs \
             || echo "Warning: some system packages failed — install them manually."
 
         # Node.js 20+: Vite needs 20.19+, but Debian/Ubuntu apt ship an EOL Node 18

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