build: upgrade to Flutter 3.47.4 (AGP 9.1 / Gradle 9.3, iOS 15 + UIScene), refresh deps and CI

Flutter 3.24.5 -> 3.47.4 with the matching toolchain changes:

- pubspec: Dart ^3.11, Firebase core 4 / auth 6 / firestore 6 / storage 13,
  drop unused image_cropper, logging, change_app_package_name,
  pdf_widget_wrapper; pin path_provider_android <2.3 (needs NDK/CMake, not
  feasible on the ARM64 Gitea runner)
- Dart: CardTheme -> CardThemeData, replace deprecated withOpacity,
  Matrix4.scale, DropdownButtonFormField.value
- iOS: deployment target 15.0, UIScene lifecycle (AppDelegate/Info.plist),
  untrack stale Podfile.lock, add LaunchImage asset set
- Android: Gradle 9.3.1, AGP 9.1.0, Kotlin 2.4.0, google-services 4.5.0,
  layout.buildDirectory instead of removed Project.buildDir, remove manifest
  package attribute and duplicate MainActivity, kotlin.incremental=false
- CI: pin FLUTTER_VERSION in both workflows; iOS on macos-26 with SwiftPM
  disabled (CocoaPods only); Gitea job keeps cirruslabs 3.44.0 as base image
  (images discontinued) and switches the bundled Flutter SDK by git tag

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

lib/screens/admin_messages_screen.dart

@@ -284,7 +284,7 @@ class _AdminMessagesScreenState extends State<AdminMessagesScreen> {
     return Expanded(
       child: OutlinedButton(
         style: OutlinedButton.styleFrom(
-          backgroundColor: isCurrent ? color.withOpacity(0.15) : null,
+          backgroundColor: isCurrent ? color.withValues(alpha: 0.15) : null,
           side: BorderSide(
             color: isCurrent ? color : Colors.grey.shade300,
             width: isCurrent ? 2 : 1,
@@ -332,7 +332,7 @@ class _AdminMessagesScreenState extends State<AdminMessagesScreen> {
       decoration: BoxDecoration(
         color: bg,
         borderRadius: BorderRadius.circular(12),
-        border: Border.all(color: fg.withOpacity(0.3)),
+        border: Border.all(color: fg.withValues(alpha: 0.3)),
       ),
       child: Text(
         status,