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

android/app/src/main/kotlin/com/example/gel_rule_app/MainActivity.kt

@@ -54,6 +54,8 @@ class MainActivity : FlutterActivity() {
                             setPreferredRefreshRate(hz.toFloat())
                             result.success(null)
                         }
+                        "getSupportedAbis" -> result.success(Build.SUPPORTED_ABIS.toList())
+                        "getPrimaryAbi" -> result.success(Build.SUPPORTED_ABIS.firstOrNull() ?: "armeabi-v7a")
                         else -> result.notImplemented()
                     }
                 } catch (error: Throwable) {