feat(programs): Android 端通过 MediaStore 写入公共 Downloads

Android 10+ 受 scoped storage 限制，getDownloadsDirectory() 返回的是
app-specific 目录 (/storage/emulated/0/Android/data/.../files/Download/)，
而非用户可见的 /storage/emulated/0/Download/。

新增 MainActivity 端 MethodChannel com.xiarui.kuaishai2/downloads：
- API 29+：MediaStore.Downloads 写入公共 Downloads，无需权限
- API <=28：直接写 /storage/emulated/0/Download/（需 WRITE_EXTERNAL_STORAGE）

Dart 端 ExcelTemplateService 改用 MethodChannel，Android 平台返回
Download/<filename> 显示路径；其它平台保留 getDownloadsDirectory 行为。
返回值由 File 改为 String，调用方已同步更新。

android/app/src/main/kotlin/com/xiarui/kuaishai2/MainActivity.kt

@@ -1,10 +1,18 @@
 package com.xiarui.kuaishai2
 
+import android.content.ContentValues
+import android.os.Build
 import android.os.Bundle
+import android.os.Environment
+import android.provider.MediaStore
 import android.view.KeyEvent
 import android.view.View
 import android.view.WindowManager
 import io.flutter.embedding.android.FlutterActivity
+import io.flutter.embedding.engine.FlutterEngine
+import io.flutter.plugin.common.MethodChannel
+import java.io.File
+import java.io.FileOutputStream
 
 /**
  * Kiosk 模式主 Activity
@@ -14,12 +22,80 @@ import io.flutter.embedding.android.FlutterActivity
  */
 class MainActivity : FlutterActivity() {
 
+    private val downloadsChannel = "com.xiarui.kuaishai2/downloads"
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         // 保持屏幕常亮
         window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
     }
 
+    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
+        MethodChannel(flutterEngine.dartExecutor.binaryMessenger, downloadsChannel)
+            .setMethodCallHandler { call, result ->
+                when (call.method) {
+                    "saveToDownloads" -> handleSaveToDownloads(call, result)
+                    else -> result.notImplemented()
+                }
+            }
+    }
+
+    private fun handleSaveToDownloads(
+        call: io.flutter.plugin.common.MethodCall,
+        result: MethodChannel.Result,
+    ) {
+        val filename = call.argument<String>("filename")
+        val bytes = call.argument<ByteArray>("bytes")
+        if (filename.isNullOrEmpty() || bytes == null) {
+            result.error("ARG_ERROR", "filename 和 bytes 必填", null)
+            return
+        }
+        try {
+            val savedPath = saveToDownloads(filename, bytes)
+            result.success(savedPath)
+        } catch (e: Exception) {
+            result.error("SAVE_FAILED", e.message ?: "保存失败", null)
+        }
+    }
+
+    /**
+     * 保存到公共 Downloads 目录。
+     * - Android 10+：通过 MediaStore.Downloads 写入，无需 WRITE_EXTERNAL_STORAGE 权限
+     * - Android 9 及以下：直接写 /storage/emulated/0/Download/，需 WRITE_EXTERNAL_STORAGE
+     */
+    private fun saveToDownloads(filename: String, bytes: ByteArray): String {
+        val mimeType =
+            "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet"
+        return if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+            val values = ContentValues().apply {
+                put(MediaStore.Downloads.DISPLAY_NAME, filename)
+                put(MediaStore.Downloads.MIME_TYPE, mimeType)
+                put(MediaStore.Downloads.IS_PENDING, 1)
+            }
+            val resolver = contentResolver
+            val uri = resolver.insert(
+                MediaStore.Downloads.EXTERNAL_CONTENT_URI,
+                values,
+            ) ?: throw IllegalStateException("无法创建 MediaStore 记录")
+            resolver.openOutputStream(uri)?.use { it.write(bytes) }
+                ?: throw IllegalStateException("无法打开输出流")
+            values.clear()
+            values.put(MediaStore.Downloads.IS_PENDING, 0)
+            resolver.update(uri, values, null, null)
+            "${Environment.DIRECTORY_DOWNLOADS}/$filename"
+        } else {
+            @Suppress("DEPRECATION")
+            val dir = Environment.getExternalStoragePublicDirectory(
+                Environment.DIRECTORY_DOWNLOADS,
+            )
+            if (!dir.exists()) dir.mkdirs()
+            val file = File(dir, filename)
+            FileOutputStream(file).use { it.write(bytes) }
+            file.absolutePath
+        }
+    }
+
     override fun onWindowFocusChanged(hasFocus: Boolean) {
         super.onWindowFocusChanged(hasFocus)
         if (hasFocus) {