feat(chat): 添加聊天界面和文件更新SDK功能

- 实现完整的聊天界面UI组件，支持文本、图片、视频、音频、文件等多种消息类型
- 集成IM消息收发功能，实现消息气泡显示和用户头像占位符
- 添加媒体文件选择和拍摄功能，支持相册图片、视频及相机拍照录像
- 实现语音录制和播放功能，包含按住说话交互和权限处理
- 添加群组提及功能，支持@用户和提及候选列表显示
- 实现消息回复和引用功能，支持消息长按回复操作
- 添加本地消息搜索功能，支持搜索当前会话的历史消息
- 实现文件上传下载功能，集成FileSDK进行文件传输管理
- 添加应用更新检查功能，集成UpdateSDK支持版本更新
- 实现消息状态显示，包括发送、送达、已读等状态标识
- 添加群组已读人数统计，显示消息在群聊中的阅读情况
- 实现草稿保存和恢复功能，支持断点续聊体验
- 添加连接状态横幅，实时显示IM服务连接状态
- 实现滚动加载更多历史消息，优化大量消息的性能表现
- 添加多媒体文件下载保存功能，支持保存到应用专属目录

sample-app/src/main/java/com/xuqm/sdk/sample/ui/chat/ChatScreen.kt

@@ -44,6 +44,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.setValue
+import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.snapshotFlow
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.layout.ContentScale
@@ -68,10 +69,10 @@ import com.xuqm.sdk.ui.SearchBarField
 import coil3.compose.AsyncImage
 import androidx.compose.ui.platform.LocalContext
 import androidx.core.content.FileProvider
+import com.xuqm.sdk.file.FileSDK
 import kotlinx.coroutines.flow.distinctUntilChanged
 import kotlinx.coroutines.launch
 import java.io.File
-import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.ui.text.input.TextFieldValue
 import androidx.compose.ui.text.TextRange
 
@@ -514,6 +515,8 @@ private fun MessageBubble(
     onReply: (ImMessage) -> Unit,
 ) {
     val media = message.mediaContent()
+    val context = LocalContext.current
+    val scope = rememberCoroutineScope()
     val arrangement = if (isOwn) Arrangement.End else Arrangement.Start
     val bubbleColor = if (isOwn) MaterialTheme.colorScheme.primaryContainer
                       else MaterialTheme.colorScheme.surfaceVariant
@@ -557,6 +560,27 @@ private fun MessageBubble(
                         style = MaterialTheme.typography.bodyMedium,
                     )
                 }
+                if (media?.url.isNullOrBlank().not() && message.msgType.uppercase() in setOf("IMAGE", "VIDEO", "AUDIO", "FILE")) {
+                    TextButton(
+                        onClick = {
+                            scope.launch {
+                                runCatching {
+                                    val saved = FileSDK.downloadToAppFiles(
+                                        context = context,
+                                        downloadUrl = media.url,
+                                        fileName = media.name?.takeIf { it.isNotBlank() } ?: defaultDownloadFileName(message),
+                                        directoryName = "demo-downloads",
+                                    )
+                                    Toast.makeText(context, "已保存到 ${saved.absolutePath}", Toast.LENGTH_SHORT).show()
+                                }.onFailure {
+                                    Toast.makeText(context, "下载失败：${it.message ?: "未知错误"}", Toast.LENGTH_SHORT).show()
+                                }
+                            }
+                        },
+                    ) {
+                        Text("下载")
+                    }
+                }
                 val mentionedUserIds = message.mentionedUserIds.orEmpty()
                 if (mentionedUserIds.isNotBlank() &&
                     mentionedUserIds.split(",").map { it.trim() }.contains(currentUserId)
@@ -765,6 +789,17 @@ private fun formatDuration(ms: Long): String {
     return if (minutes > 0) String.format("%d:%02d", minutes, seconds) else "${seconds}s"
 }
 
+private fun defaultDownloadFileName(message: ImMessage): String {
+    val suffix = when (message.msgType.uppercase()) {
+        "IMAGE" -> "jpg"
+        "VIDEO" -> "mp4"
+        "AUDIO" -> "m4a"
+        "FILE" -> "bin"
+        else -> "dat"
+    }
+    return "${message.msgType.lowercase()}_${message.id}.$suffix"
+}
+
 private fun mentionQueryAtCursor(value: TextFieldValue): String? {
     if (!value.selection.collapsed) return null
     val cursor = value.selection.end.coerceIn(0, value.text.length)

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

@@ -4,12 +4,15 @@ import android.content.Context
 import android.net.Uri
 import com.xuqm.sdk.core.ServiceEndpointRegistry
 import com.xuqm.sdk.network.ApiClient
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
 import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.MultipartBody
 import okhttp3.RequestBody.Companion.toRequestBody
 import retrofit2.http.Multipart
 import retrofit2.http.POST
 import retrofit2.http.Part
+import java.io.File
 
 data class FileUploadResult(
     val url: String,
@@ -90,4 +93,31 @@ object FileSDK {
             "File upload failed"
         }
     }
+
+    suspend fun downloadToFile(
+        downloadUrl: String,
+        targetFile: File,
+        onProgress: (Int) -> Unit = {},
+    ): File = withContext(Dispatchers.IO) {
+        FileTransfer.downloadToFile(downloadUrl, targetFile, onProgress)
+        targetFile
+    }
+
+    suspend fun downloadToAppFiles(
+        context: Context,
+        downloadUrl: String,
+        fileName: String? = null,
+        directoryName: String? = null,
+        onProgress: (Int) -> Unit = {},
+    ): File = withContext(Dispatchers.IO) {
+        val dir = if (directoryName.isNullOrBlank()) {
+            context.getExternalFilesDir(null) ?: context.filesDir
+        } else {
+            File(context.getExternalFilesDir(null) ?: context.filesDir, directoryName).apply { mkdirs() }
+        }
+        val resolvedName = fileName?.takeIf { it.isNotBlank() } ?: downloadUrl.substringAfterLast('/').takeIf { it.isNotBlank() } ?: "download.bin"
+        val target = File(dir, resolvedName)
+        FileTransfer.downloadToFile(downloadUrl, target, onProgress)
+        target
+    }
 }

sdk-update/src/main/java/com/xuqm/sdk/update/UpdateSDK.kt

@@ -5,7 +5,7 @@ import android.content.Intent
 import android.net.Uri
 import androidx.core.content.FileProvider
 import com.xuqm.sdk.XuqmSDK
-import com.xuqm.sdk.file.FileTransfer
+import com.xuqm.sdk.file.FileSDK
 import com.xuqm.sdk.core.ServiceEndpointRegistry
 import com.xuqm.sdk.network.ApiClient
 import com.xuqm.sdk.update.api.UpdateApi
@@ -53,7 +53,7 @@ object UpdateSDK {
         onProgress: (Int) -> Unit = {},
     ) = withContext(Dispatchers.IO) {
         val apkFile = File(context.getExternalFilesDir(null), "update.apk")
-        FileTransfer.downloadToFile(downloadUrl, apkFile, onProgress)
+        FileSDK.downloadToFile(downloadUrl, apkFile, onProgress)
         withContext(Dispatchers.Main) { installApk(context, apkFile) }
     }