Add in-app update checker and one-click updater

Settings now shows when a newer version is available, with the real
version number read from the remote's package.json (via the git
binary already required for this deployment model, so it works
against any remote, not just GitHub), and an "Uppdatera nu" button
that stops the server, pulls, reinstalls dependencies and restarts
automatically - warning explicitly if a race is currently active.
The existing app-version poller already reloads the page once a
restart is detected, so no new client-side polling was needed.

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "rc-racecore",
-  "version": "1.7.1",
+  "version": "1.8.0",
   "private": true,
   "description": "RC RaceCore live timing and race control with AMMC ingest and SQLite persistence",
   "engines": {
@@ -18,6 +18,7 @@
     "stop": "node scripts/serverctl.js stop",
     "status": "node scripts/serverctl.js status",
     "restart": "node scripts/serverctl.js restart",
+    "update": "node scripts/serverctl.js update",
     "verify": "node scripts/verify-install.js",
     "backup:restore": "node scripts/restore-backup.js",
     "audit:prod": "npm audit --omit=dev",