Add systemd install script and improve reset-site.sh resilience

- Add scripts/systemd/install.sh to handle placeholder substitution
  (__PROJECT_DIR__, __USER__) and systemd unit installation in one command
- Simplify manual install instructions in SettingsPage and config.sh to
  reference the new install script
- Preserve existing home.html and home.css in reset-site.sh instead of
  overwriting with templates
- Add comments/ and partials/ to preserved directories list
- Fix nav removal in mkdocs.yml using Python regex instead of fragile sed

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/systemd/install.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# =============================================================================
+# Install Changemaker Lite systemd units
+# Substitutes __PROJECT_DIR__ and __USER__ placeholders with actual values.
+# Usage: sudo ./scripts/systemd/install.sh
+# =============================================================================
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_DIR="$(dirname "$(dirname "$SCRIPT_DIR")")"
+INSTALL_USER="${SUDO_USER:-$(whoami)}"
+
+if [[ $EUID -ne 0 ]]; then
+  echo "ERROR: Must run as root (use sudo)." >&2
+  exit 1
+fi
+
+echo "Installing systemd units..."
+echo "  Project dir: ${PROJECT_DIR}"
+echo "  User:        ${INSTALL_USER}"
+
+for unit in "${SCRIPT_DIR}"/changemaker-upgrade.*; do
+  filename="$(basename "$unit")"
+  sed \
+    -e "s|__PROJECT_DIR__|${PROJECT_DIR}|g" \
+    -e "s|__USER__|${INSTALL_USER}|g" \
+    "$unit" > "/etc/systemd/system/${filename}"
+  echo "  Installed ${filename}"
+done
+
+systemctl daemon-reload
+systemctl enable --now changemaker-upgrade.path
+
+echo "Done. Status:"
+systemctl status changemaker-upgrade.path --no-pager