fix: preserve node_modules during upgrade to avoid re-downloading Electron

The upgrade was wiping node_modules then relying on npm to re-download the
~120MB Electron binary. npm returns exit 0 even when the download times out,
leaving the kiosk with no Electron binary and a blank screen on next boot.

node_modules only needs to be deleted on a fresh install or when explicitly
changing Electron version. For a JS-file-only upgrade, npm install without
a wipe is either a no-op (no changes) or applies dependency updates cleanly.
The install_electron_binary fallback remains as a safety net.

https://claude.ai/code/session_01EyjEQLWbTXcZgbMDarf7NU

ubuntu-based-kiosk-v1.0.2.sh

@@ -11093,8 +11093,8 @@ upgrade_kiosk() {
     sudo rm -f "$KIOSK_DIR/pause-dialog.html"
     sudo rm -f "$KIOSK_DIR/pin-entry.html"
     sudo rm -f "$KIOSK_DIR/inactivity-prompt-extended.html"
-    sudo rm -rf "$KIOSK_DIR/node_modules"
-    sudo rm -f "$KIOSK_DIR/package-lock.json"
+    # node_modules is NOT removed — preserves the Electron binary (~120MB download)
+    # npm install below is a no-op if dependencies haven't changed
     log_success "Old files removed"
 
     echo "[4/6] Extracting new app files from script..."