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>

android/settings.gradle

@@ -20,11 +20,14 @@ pluginManagement {
     }
 }
 
+// Versions follow the Flutter 3.47 project template (see
+// packages/flutter_tools/lib/src/android/gradle_utils.dart in the Flutter SDK).
+// Flutter refuses to build with AGP < 8.11.1, Kotlin < 2.2.20 or Gradle < 8.14.
 plugins {
     id "dev.flutter.flutter-plugin-loader" version "1.0.0"
-    id "com.android.application" version "8.5.2" apply false
-    id "org.jetbrains.kotlin.android" version "1.9.22" apply false
-    id "com.google.gms.google-services" version "4.4.1" apply false
+    id "com.android.application" version "9.1.0" apply false
+    id "org.jetbrains.kotlin.android" version "2.4.0" apply false
+    id "com.google.gms.google-services" version "4.5.0" apply false
 }
 
 include ":app"