feat: ABI-aware auto-update — pick correct per-ABI APK from Gitea

- Android: expose getSupportedAbis/getPrimaryAbi via rulegel/device channel (Build.SUPPORTED_ABIS)
- AppUpdateInfo: add apkArm64Url, apkArmv7Url, apkX86_64Url fields + apkUrlForAbi() selector
- UpdateService: parse per-ABI APK assets by filename pattern, async assetUrlForCurrentPlatform
- app.dart + settings_screen.dart: await new async URL resolver
- Bump version to 2.3.8+48

lib/backend/models/app_update_info.dart

@@ -7,6 +7,9 @@ class AppUpdateInfo {
     required this.htmlUrl,
     required this.publishedAt,
     this.apkUrl,
+    this.apkArm64Url,
+    this.apkArmv7Url,
+    this.apkX86_64Url,
     this.windowsInstallerUrl,
     this.portableZipUrl,
     this.linuxTarGzUrl,
@@ -18,13 +21,36 @@ class AppUpdateInfo {
   final String body;
   final String htmlUrl;
   final DateTime? publishedAt;
+
+  /// Generic APK (fat / universal build). Used as a fallback.
   final String? apkUrl;
+
+  /// Per-ABI APKs produced by `flutter build apk --split-per-abi`.
+  final String? apkArm64Url;   // app-arm64-v8a-release.apk
+  final String? apkArmv7Url;   // app-armeabi-v7a-release.apk
+  final String? apkX86_64Url;  // app-x86_64-release.apk
+
   final String? windowsInstallerUrl;
   final String? portableZipUrl;
   final String? linuxTarGzUrl;
 
+  /// Returns the best APK URL for [abi], falling back to the generic APK.
+  String? apkUrlForAbi(String abi) {
+    if (abi.contains('arm64') || abi == 'arm64-v8a') {
+      return apkArm64Url ?? apkUrl;
+    }
+    if (abi.contains('x86_64')) {
+      return apkX86_64Url ?? apkUrl;
+    }
+    // armeabi-v7a and everything else
+    return apkArmv7Url ?? apkArm64Url ?? apkUrl;
+  }
+
   bool get hasAssets =>
       apkUrl != null ||
+      apkArm64Url != null ||
+      apkArmv7Url != null ||
+      apkX86_64Url != null ||
       windowsInstallerUrl != null ||
       portableZipUrl != null ||
       linuxTarGzUrl != null;