Added onboarding + a lightweight install banner to both the mobile login screen and the settings screen, with Android/Chromium

install prompt support and iOS “Share → Add to Home Screen” guidance. Also added a small helper + tests to decide
  when/which banner variant should show, and shared copy in common.json.

resources/js/admin/i18n/locales/en/common.json

@@ -66,6 +66,12 @@
     "viewAll": "View all",
     "dismiss": "Dismiss"
   },
+  "installBanner": {
+    "title": "Install Fotospiel Admin",
+    "body": "Add the app to your home screen for faster access and offline support.",
+    "action": "Install",
+    "iosHint": "On iOS: Share → Add to Home Screen."
+  },
   "errors": {
     "generic": "Something went wrong. Please try again.",
     "eventLimit": "Your current package has no remaining event slots.",