feat: 添加图标大小限制和无缓存请求头，优化 WebView 加载逻辑

android/app/src/main/kotlin/io/openim/flutter/openim/MainActivity.kt

@@ -24,6 +24,7 @@ class MainActivity : FlutterActivity() {
     private val CHANNEL = "io.openim.flutter.openim/apk_info"
     private val SHELL_BRANDING_CHANNEL = "io.openim.flutter.im_webview_app/shell_branding"
     private val TAG = "MainActivity"
+    private val MAX_BRANDING_ICON_SIZE = 192
 
     override fun onCreate(savedInstanceState: android.os.Bundle?) {
         // 华为/荣耀/OPPO 等国产设备：在任意网络请求之前同步安装 Conscrypt，修复 SSL 握手失败（无 GMS 时系统 SSL 实现不完整）
@@ -115,13 +116,27 @@ class MainActivity : FlutterActivity() {
 
     private fun getCurrentAppIconDataUrl(): String {
         val icon = packageManager.getApplicationIcon(packageName)
-        val bitmap = drawableToBitmap(icon)
+        val bitmap = resizeBitmap(drawableToBitmap(icon), MAX_BRANDING_ICON_SIZE)
         val output = ByteArrayOutputStream()
         bitmap.compress(Bitmap.CompressFormat.PNG, 100, output)
         val base64 = Base64.encodeToString(output.toByteArray(), Base64.NO_WRAP)
         return "data:image/png;base64,$base64"
     }
 
+    private fun resizeBitmap(bitmap: Bitmap, maxSize: Int): Bitmap {
+        val width = bitmap.width
+        val height = bitmap.height
+        val longestSide = maxOf(width, height)
+        if (longestSide <= maxSize) {
+            return bitmap
+        }
+
+        val scale = maxSize.toFloat() / longestSide.toFloat()
+        val scaledWidth = (width * scale).toInt().coerceAtLeast(1)
+        val scaledHeight = (height * scale).toInt().coerceAtLeast(1)
+        return Bitmap.createScaledBitmap(bitmap, scaledWidth, scaledHeight, true)
+    }
+
     private fun drawableToBitmap(drawable: Drawable): Bitmap {
         if (drawable is BitmapDrawable && drawable.bitmap != null) {
             return drawable.bitmap