build: pin AGP 8.7.3, and generate the flame launcher icon

The release build failed while evaluating flutter_inappwebview_android:

  getDefaultProguardFile('proguard-android.txt') is no longer supported

AGP 9 removed that API. The plugin still calls it at configuration time, in
both its debug and release buildTypes, so the failure happens whether or not
minification is on. 1.1.3 is the newest release of that plugin — 6.1.5 is the
current top-level package — so there is nothing to upgrade to; the template
simply pinned an AGP ahead of the plugin ecosystem.

  com.android.application  9.1.0 -> 8.7.3
  kotlin.android           2.4.0 -> 2.1.0   (matched to AGP 8.x)
  gradle                   9.3.1 -> 8.12    (AGP 8.7 does not support Gradle 9)

Fixed here rather than by patching ~/.pub-cache: that cache is generated, shared
between projects, outside git, and erased by `pub cache repair`. A fix there
works on exactly one machine and silently disappears — including on the server,
once Flutter moves into the Docker image. Reverse this pin when the plugin ships
an AGP 9 build.

The launcher icon is generated by a Pillow script from the same curve geometry
as the in-app FlameMark painter, so the icon and the mark cannot drift apart.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

android/gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-9.3.1-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-8.12-all.zip

android/settings.gradle.kts

@@ -19,8 +19,11 @@ pluginManagement {
 
 plugins {
     id("dev.flutter.flutter-plugin-loader") version "1.0.0"
-    id("com.android.application") version "9.1.0" apply false
-    id("org.jetbrains.kotlin.android") version "2.4.0" apply false
+    // AGP 9 removed getDefaultProguardFile("proguard-android.txt"), which
+    // flutter_inappwebview_android 1.1.3 (the newest release) still calls at
+    // configuration time. Pinned to the last 8.x until the plugin moves.
+    id("com.android.application") version "8.7.3" apply false
+    id("org.jetbrains.kotlin.android") version "2.1.0" apply false
 }
 
 include(":app")