feat(file): 添加文件下载权限管理和回退机制

- 实现 PublicDownloads 目录失败时自动回退到 Sandbox 目录
- 添加 Android 11+ 所有文件访问权限检查功能
- 提供跳转到文件访问权限设置页面的 Intent 方法
- 重构目录解析逻辑为独立的 resolveBaseDir 函数
- 增强文件下载的异常处理和日志记录
- 修复空白文件名处理逻辑，确保默认文件名为 download.bin

sdk-core/src/main/java/com/xuqm/sdk/file/FileSDK.kt

@@ -254,7 +254,28 @@ object FileSDK {
         android.util.Log.d("XWV", "saveBlobDownload: fileName=$fileName, dest=$destination, b64Len=${base64Data.length}")
         val bytes = android.util.Base64.decode(base64Data, android.util.Base64.DEFAULT)
         android.util.Log.d("XWV", "saveBlobDownload: decoded ${bytes.size} bytes")
-        val baseDir = when (destination) {
+
+        val safeName = fileName.takeIf { it.isNotBlank() } ?: "download.bin"
+
+        // 尝试目标目录，如果 PublicDownloads 失败则回退到 Sandbox
+        val baseDir = resolveBaseDir(context, destination)
+        val target = uniqueFile(baseDir, safeName)
+        android.util.Log.d("XWV", "saveBlobDownload: writing to ${target.absolutePath}")
+        return try {
+            target.writeBytes(bytes)
+            android.util.Log.d("XWV", "saveBlobDownload: done, size=${target.length()}")
+            target
+        } catch (e: java.io.IOException) {
+            android.util.Log.w("XWV", "saveBlobDownload: primary dir failed (${e.message}), falling back to Sandbox")
+            val fallbackDir = resolveBaseDir(context, FileDownloadDestination.Sandbox)
+            val fallbackTarget = uniqueFile(fallbackDir, safeName)
+            fallbackTarget.writeBytes(bytes)
+            android.util.Log.d("XWV", "saveBlobDownload: fallback done, ${fallbackTarget.absolutePath}, size=${fallbackTarget.length()}")
+            fallbackTarget
+        }
+    }
+
+    private fun resolveBaseDir(context: Context, destination: FileDownloadDestination): File = when (destination) {
         FileDownloadDestination.PublicDownloads -> {
             val appName = context.applicationInfo.loadLabel(context.packageManager).toString()
             File(
@@ -262,22 +283,39 @@ object FileSDK {
                     android.os.Environment.DIRECTORY_DOWNLOADS
                 ),
                 appName,
-                ).apply {
-                    val created = mkdirs()
-                    android.util.Log.d("XWV", "saveBlobDownload: PublicDownloads dir=$absolutePath, created=$created, exists=${exists()}")
-                }
+            ).apply { mkdirs() }
         }
         FileDownloadDestination.Sandbox -> {
-                (context.getExternalFilesDir(null) ?: context.filesDir).also {
-                    android.util.Log.d("XWV", "saveBlobDownload: Sandbox dir=${it.absolutePath}")
+            context.getExternalFilesDir(null) ?: context.filesDir
         }
     }
+
+    /**
+     * 检查是否拥有"所有文件访问权限"（MANAGE_EXTERNAL_STORAGE）。
+     * Android 11+ 需要此权限才能写入公共 Downloads 目录。
+     * Android 10 及以下始终返回 true（使用 WRITE_EXTERNAL_STORAGE）。
+     */
+    fun isManageStorageGranted(context: Context): Boolean {
+        return if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+            Environment.isExternalStorageManager()
+        } else {
+            true
+        }
+    }
+
+    /**
+     * 返回跳转到"所有文件访问权限"设置页的 Intent。
+     * 调用方应使用 startActivityForResult 或 registerForActivityResult 处理返回。
+     */
+    fun requestManageStorageIntent(context: Context): Intent? {
+        if (Build.VERSION.SDK_INT < Build.VERSION_CODES.R) return null
+        return try {
+            Intent(android.provider.Settings.ACTION_MANAGE_APP_ALL_FILES_ACCESS_PERMISSION).apply {
+                data = Uri.parse("package:${context.packageName}")
+            }
+        } catch (_: Exception) {
+            Intent(android.provider.Settings.ACTION_MANAGE_ALL_FILES_ACCESS_PERMISSION)
         }
-        val target = uniqueFile(baseDir, fileName.takeIf { it.isNotBlank() } ?: "download.bin")
-        android.util.Log.d("XWV", "saveBlobDownload: writing to ${target.absolutePath}")
-        target.writeBytes(bytes)
-        android.util.Log.d("XWV", "saveBlobDownload: done, size=${target.length()}")
-        return target
     }
 
     /**