feat(install): add proper update/migration support for existing installations

Enhanced install.sh to properly handle updates and migrations:

**Update Detection & Service Management:**
- Detect old Node.js-based installations (v3.x) via package.json/node_modules
- Stop running service before updating binary
- Restart service after successful update if it was running
- Preserve /etc/nupst/config.json during updates

**Migration from v3.x to v4.0:**
- Clean up old Node.js installation files:
  - node_modules/, vendor/, dist_ts/ directories
  - package.json, package-lock.json, pnpm-lock.yaml
  - tsconfig.json, setup.sh, bin/ directory
- Inform user about migration with helpful feedback
- Link to migration guide documentation

**User Experience Improvements:**
- Show different messages for new installs vs updates
- Inform about v3.x → v4.0 migration when detected
- Display migration guide link for old installations
- Show service restart status
- Provide context-aware next steps based on config presence

**Safety Features:**
- Ask for confirmation before replacing existing installation (interactive mode)
- Preserve user configuration in /etc/nupst/
- Handle service state properly (stop → update → restart)
- Graceful cleanup with error suppression (|| true)

This ensures seamless updates from any version (including v3.x Node.js installs)
to v4.0+ Deno-based binaries without manual intervention or data loss.

install.sh

@@ -213,8 +213,24 @@ echo "Download URL: $DOWNLOAD_URL"
 echo ""
 
 # Check if installation directory exists
+SERVICE_WAS_RUNNING=0
+OLD_NODE_INSTALL=0
+
 if [ -d "$INSTALL_DIR" ]; then
+  # Check if this is an old Node.js-based installation
+  if [ -f "$INSTALL_DIR/package.json" ] || [ -d "$INSTALL_DIR/node_modules" ]; then
+    OLD_NODE_INSTALL=1
+    echo "Detected old Node.js-based NUPST installation (v3.x or earlier)"
+    echo "This installer will migrate to the new Deno-based binary version (v4.0+)"
+    echo ""
+  fi
+
   if [ $AUTO_YES -eq 0 ] && [ $INTERACTIVE -eq 1 ]; then
+    if [ $OLD_NODE_INSTALL -eq 1 ]; then
+      echo "This will replace your Node.js installation with a pre-compiled binary."
+      echo "Your configuration in /etc/nupst/config.json will be preserved."
+      echo ""
+    fi
     echo "Installation directory already exists: $INSTALL_DIR"
     echo "Do you want to update/reinstall? (Y/n): "
     read -r update_confirm
@@ -226,6 +242,28 @@ if [ -d "$INSTALL_DIR" ]; then
   fi
 
   echo "Updating existing installation at $INSTALL_DIR..."
+
+  # Check if service is running and stop it
+  if systemctl is-active --quiet nupst 2>/dev/null; then
+    echo "Stopping NUPST service..."
+    systemctl stop nupst
+    SERVICE_WAS_RUNNING=1
+  fi
+
+  # Clean up old Node.js installation files
+  if [ $OLD_NODE_INSTALL -eq 1 ]; then
+    echo "Cleaning up old Node.js installation files..."
+    rm -rf "$INSTALL_DIR/node_modules" 2>/dev/null || true
+    rm -rf "$INSTALL_DIR/vendor" 2>/dev/null || true
+    rm -rf "$INSTALL_DIR/dist_ts" 2>/dev/null || true
+    rm -f "$INSTALL_DIR/package.json" 2>/dev/null || true
+    rm -f "$INSTALL_DIR/package-lock.json" 2>/dev/null || true
+    rm -f "$INSTALL_DIR/pnpm-lock.yaml" 2>/dev/null || true
+    rm -f "$INSTALL_DIR/tsconfig.json" 2>/dev/null || true
+    rm -f "$INSTALL_DIR/setup.sh" 2>/dev/null || true
+    rm -rf "$INSTALL_DIR/bin" 2>/dev/null || true
+    echo "Old installation files removed."
+  fi
 else
   if [ $AUTO_YES -eq 0 ] && [ $INTERACTIVE -eq 1 ]; then
     echo "NUPST will be installed to: $INSTALL_DIR"
@@ -301,18 +339,54 @@ else
 fi
 
 echo ""
+
+# Restart service if it was running before update
+if [ $SERVICE_WAS_RUNNING -eq 1 ]; then
+  echo "Restarting NUPST service..."
+  systemctl start nupst
+  echo "Service restarted successfully."
+  echo ""
+fi
+
 echo "================================================"
 echo "  NUPST Installation Complete!"
 echo "================================================"
 echo ""
+
+if [ $OLD_NODE_INSTALL -eq 1 ]; then
+  echo "Migration from v3.x to v4.0 successful!"
+  echo ""
+  echo "What changed:"
+  echo "  • Node.js runtime removed (now a self-contained binary)"
+  echo "  • Faster startup and lower memory usage"
+  echo "  • CLI commands now use subcommand structure"
+  echo "    (old commands still work with deprecation warnings)"
+  echo ""
+  echo "Migration guide: https://code.foss.global/serve.zone/nupst/src/branch/main/MIGRATION.md"
+  echo ""
+fi
+
 echo "Installation details:"
 echo "  Binary location: $BINARY_PATH"
 echo "  Symlink location: $BIN_DIR/nupst"
 echo "  Version: $VERSION"
 echo ""
-echo "Get started:"
+
-echo "  nupst --version"
+# Check if configuration exists
-echo "  nupst help"
+if [ -f "/etc/nupst/config.json" ]; then
-echo "  nupst ups add       # Add a UPS device"
+  echo "Configuration: /etc/nupst/config.json (preserved)"
-echo "  nupst service enable # Enable systemd service"
+  echo ""
+  echo "Your existing configuration has been preserved."
+  if [ $SERVICE_WAS_RUNNING -eq 1 ]; then
+    echo "The service has been restarted with your current settings."
+  else
+    echo "Start the service with: sudo nupst service start"
+  fi
+else
+  echo "Get started:"
+  echo "  nupst --version"
+  echo "  nupst help"
+  echo "  nupst ups add       # Add a UPS device"
+  echo "  nupst service enable # Enable systemd service"
+fi
 echo ""