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

pubspec.lock

@@ -1197,6 +1197,22 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "2.2.0"
+  package_info_plus:
+    dependency: transitive
+    description:
+      name: package_info_plus
+      sha256: "16eee997588c60225bda0488b6dcfac69280a6b7a3cf02c741895dd370a02968"
+      url: "https://pub.dev"
+    source: hosted
+    version: "8.3.1"
+  package_info_plus_platform_interface:
+    dependency: transitive
+    description:
+      name: package_info_plus_platform_interface
+      sha256: "202a487f08836a592a6bd4f901ac69b3a8f146af552bbd14407b6b41e1c3f086"
+      url: "https://pub.dev"
+    source: hosted
+    version: "3.2.1"
   path:
     dependency: transitive
     description: