feat: version update check — notify when newer version is on GitHub (v0.11.8 Beta)

Backend: GET /api/system/version-check fetches frontend/package.json from
the main branch on GitHub, caches result in app_settings for 24 hours,
and compares semver against the running version. Fails silently (no banner)
for air-gapped or firewalled deployments.

Frontend: dismissible blue update banner in SecurityBanners (rendered
between TopBar and main content) — only visible when a newer version is
detected. Shows target version and docker compose update command.

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mikrotik-manager-frontend",
-  "version": "0.11.7-beta",
+  "version": "0.11.8-beta",
   "private": true,
   "type": "module",
   "scripts": {