feat(version-check): add in-app update prompt with Remote Config

Adds a production-grade in-app version check that prompts users to update when a new build is available. Soft updates are dismissable. Force updates block the app entirely. Configured via FirebaseRemote Config so rollouts can be triggered without redeploying.

- Sealed result types (NoUpdate / SoftUpdate / ForceUpdate) for type-safe pattern matching
- AppVersionCheckService
- AppUpdateGate widget encapsulates listener, route guard and dialog wireup, isolated from save_family_app
- Serialized notifier operations prevent race between dismiss and refresh, mounted checks blindar disposal edge cases                                                                                                  - Build-aware dismiss persistence via SharedPreferences

packages/sf_localizations/assets/l10n/de.json

@@ -857,5 +857,11 @@
   "videoCall": "Videoanruf",
   "watchesOnMap": "Smartwatches auf der Karte:",
   "wifiSettings": "WLAN-Einstellungen",
-  "yesterday": "Gestern"
+  "yesterday": "Gestern",
+  "appUpdateAvailableTitle": "Update verfügbar",
+  "appUpdateAvailableMessage": "Eine neue Version von SaveFamily ist verfügbar.",
+  "appUpdateRequiredTitle": "Update erforderlich",
+  "appUpdateRequiredMessage": "Du musst SaveFamily aktualisieren, um die App weiterhin zu nutzen.",
+  "appUpdateLater": "Später",
+  "appUpdateNow": "Jetzt aktualisieren"
 }