feature: media auto-download + manual download + UI download state indication (Android + desktop)

C-layer:
- chat_event.h: CHAT_EVT_ATTACHMENT_DOWNLOADED (14)
- chat_msg.c: fixed md_auto_download (non-NULL done_cb, correct media_base from db_path, use chat_setting for autoload checks)
- chat_msg.c: added chat_core_attachment_download + trampoline for manual download
- chat_core.h: exported attachment_dl_req + trampoline

Android (Kotlin + JNI):
- Message: downloadState field (parse from localAttrs)
- ChatRepository: parse localAttrs for download state
- NativeLib: attachmentDownload() JNI
- MessageBubble: FileBubble with download status icon + tap-to-download
- ChatViewModel: downloadAttachment() + CHAT_EVT_ATTACHMENT_DOWNLOADED handler
- ChatScreen: wire onDownloadFile handler
- android_jni_bridge: utun_bridge_attachment_download + JNI + localAttrs in JSON

Desktop (Qt/C++):
- messagedelegate.h: MsgFileDownloadStateRole, MsgChannelIdRole
- messagelist.cpp: parse localAttrs for file download state
- messagedelegate.cpp: FileBubble with download icon/text + click handler via chat_core_attachment_download_trampoline
- gui_bridge.h: GUI_EVT_ATTACHMENT_DOWNLOADED (14) + callback typedef
- gui_bridge_impl.cpp: event dispatch + setter
- mainwindow.cpp: callback registration -> refresh message list

tools/chatgui-android/app/src/main/java/com/utun/chat/data/ChatRepository.kt

@@ -9,7 +9,8 @@ data class Channel(val id: String, val name: String, val lastMsgAt: Long = 0, va
 data class Message(val id: Long, val author: String, val text: String, val ts: Long,
                    val isOutgoing: Boolean = false, val contentType: String = "text/plain",
                    val filePath: String = "", val waveform: List<Float> = emptyList(),
-                   val voiceDurationMs: Int = 0, val fileSize: Long = 0)
+                   val voiceDurationMs: Int = 0, val fileSize: Long = 0,
+                   val downloadState: String = "" /* "", "downloading", "downloaded", "error" */)
 
 class ChatRepository {
     private val myNodeId: Long = NativeLib.getMyNodeId()
@@ -56,6 +57,12 @@ class ChatRepository {
                 var displayText = text
                 var fileSize = 0L
 
+                /* Parse download state from localAttrs */
+                var downloadState = ""
+                val localAttrs = obj.optString("localAttrs", "")
+                if (filePath.isNotEmpty()) downloadState = "downloaded"
+                else if (localAttrs.contains("\"st\":\"fl\"")) downloadState = "downloaded"
+
                 /* Parse base64 waveform from voice message text (format: "wf=<100 chars A-Za-z0-9+/>;dur=1234;") */
                 var waveform = emptyList<Float>()
                 var voiceDurationMs = 0
@@ -114,7 +121,8 @@ class ChatRepository {
                     filePath = filePath,
                     waveform = waveform,
                     voiceDurationMs = voiceDurationMs,
-                    fileSize = fileSize
+                    fileSize = fileSize,
+                    downloadState = downloadState
                 ))
             }
         } catch (e: Exception) { Log.w("utun-gui", "getMessages error", e) }

tools/chatgui-android/app/src/main/java/com/utun/chat/data/NativeLib.kt

@@ -99,6 +99,7 @@ object NativeLib {
 
     /* ── Attachment ── */
     fun attachmentSend(channelId: String, filePath: String): Boolean = nativeAttachmentSend(channelId, filePath)
+    fun attachmentDownload(channelId: String, msgId: Long): Boolean = nativeAttachmentDownload(channelId, msgId)
 
     /* ── Voice playback ── */
     fun voiceDecodeOpen(filePath: String, outInfo: IntArray): Long = nativeVoiceDecodeOpen(filePath, outInfo)
@@ -147,6 +148,7 @@ object NativeLib {
 
     /* ── Attachment JNI ── */
     private external fun nativeAttachmentSend(channelId: String, filePath: String): Boolean
+    private external fun nativeAttachmentDownload(channelId: String, msgId: Long): Boolean
 
     /* ── Voice playback JNI ── */
     private external fun nativeVoiceDecodeOpen(filePath: String, outInfo: IntArray): Long

tools/chatgui-android/app/src/main/java/com/utun/chat/ui/components/MessageBubble.kt

@@ -35,7 +35,8 @@ import kotlinx.coroutines.delay
 @Composable
 fun MessageBubble(message: Message, playbackState: ChatViewModel.PlaybackState = ChatViewModel.PlaybackState(),
                    onPlayVoice: (String, Int) -> Unit = { _, _ -> },
-                   onSeekVoice: (String, Int, Int) -> Unit = { _, _, _ -> }) {
+                   onSeekVoice: (String, Int, Int) -> Unit = { _, _, _ -> },
+                   onDownloadFile: (Message) -> Unit = {}) {
     val bgColor = if (message.isOutgoing) Color(0xFF4A90D9) else Color(0xFFE8E8E8)
     val textColor = if (message.isOutgoing) Color.White else Color.Black
     val alignment = if (message.isOutgoing) Arrangement.End else Arrangement.Start
@@ -62,7 +63,7 @@ fun MessageBubble(message: Message, playbackState: ChatViewModel.PlaybackState =
 
             when {
                 isVoice -> VoiceBubble(message, textColor, isActive, playbackState, onPlayVoice, onSeekVoice)
-                isMedia -> FileBubble(message, textColor)
+                isMedia -> FileBubble(message, textColor, onDownloadFile)
                 else -> {
                     Text(message.author, fontSize = 13.sp, fontWeight = FontWeight.Bold, color = textColor.copy(alpha = 0.7f))
                     Spacer(Modifier.height(2.dp))
@@ -168,11 +169,33 @@ private fun VoiceBubble(message: Message, textColor: Color, isActive: Boolean,
 }
 
 @Composable
-private fun FileBubble(message: Message, textColor: Color) {
-    Column {
-        Text(message.text, fontSize = 15.sp, color = textColor, fontWeight = FontWeight.Bold)
+private fun FileBubble(message: Message, textColor: Color, onDownload: (Message) -> Unit = {}) {
+    val state = message.downloadState
+    val stateIcon = when {
+        state == "downloaded" -> "\u2705"
+        state == "downloading" -> "\u23F3"
+        else -> "\u2B07\uFE0F"
+    }
+    val stateText = when {
+        state == "downloaded" -> "Downloaded"
+        state == "downloading" -> "Downloading..."
+        else -> "Tap to download"
+    }
+
+    Column(
+        modifier = Modifier.clickable { if (state.isEmpty()) onDownload(message) }
+    ) {
+        Row(verticalAlignment = Alignment.CenterVertically) {
+            Text(stateIcon, fontSize = 18.sp)
+            Spacer(Modifier.width(6.dp))
+            Text(message.text, fontSize = 15.sp, color = textColor, fontWeight = FontWeight.Bold)
+        }
         Spacer(Modifier.height(2.dp))
-        Text(formatFileSize(message.fileSize), fontSize = 12.sp, color = textColor.copy(alpha = 0.5f))
+        Text(
+            "${formatFileSize(message.fileSize)} · $stateText",
+            fontSize = 12.sp,
+            color = textColor.copy(alpha = 0.5f)
+        )
     }
 }
 

tools/chatgui-android/app/src/main/java/com/utun/chat/ui/screens/ChatScreen.kt

@@ -66,12 +66,9 @@ fun ChatScreen(
         ) {
             items(messages.size) { idx ->
                 MessageBubble(messages[idx], playbackState = playbackState,
-                    onPlayVoice = { path, durMs ->
-                        viewModel.playVoiceMessage(path, durMs)
-                    },
-                    onSeekVoice = { path, seekMs, durMs ->
-                        viewModel.seekVoice(path, seekMs, durMs)
-                    })
+                    onPlayVoice = { path, durMs -> viewModel.playVoiceMessage(path, durMs) },
+                    onSeekVoice = { path, seekMs, durMs -> viewModel.seekVoice(path, seekMs, durMs) },
+                    onDownloadFile = { msg -> viewModel.downloadAttachment(msg) })
             }
             item { Spacer(Modifier.height(8.dp)) }
         }

tools/chatgui-android/app/src/main/java/com/utun/chat/viewmodel/ChatViewModel.kt

@@ -123,6 +123,16 @@ class ChatViewModel : ViewModel() {
                 }
             }
             8 -> if (repo != null) refreshChannels()  /* CHANNEL_PEERS_ONLINE */
+            14 -> { /* ATTACHMENT_DOWNLOADED: [ch_id_len:1][ch_id:var][msg_id:8] */
+                if (repo == null || data == null || data.size < 2) return
+                val chLen = data[0].toInt() and 0xFF
+                if (data.size >= 1 + chLen + 8) {
+                    val chId = String(data, 1, chLen)
+                    val cur = _currentChannel.value
+                    if (cur != null && cur.id == chId) refreshMessages(chId)
+                    refreshChannels()
+                }
+            }
             2 -> { /* CONNECT_RESULT: [node_id:8][result:4][channel_id:8] */
                 if (repo == null || data == null || data.size < 20) return
                 val buf = java.nio.ByteBuffer.wrap(data).order(java.nio.ByteOrder.LITTLE_ENDIAN)
@@ -279,6 +289,15 @@ class ChatViewModel : ViewModel() {
         }
     }
 
+    fun downloadAttachment(msg: Message) {
+        val ch = _currentChannel.value ?: return
+        NativeLib.attachmentDownload(ch.id, msg.id)
+        viewModelScope.launch {
+            delay(500)
+            refreshMessages(ch.id)
+        }
+    }
+
     fun getVoicePreset(): Int = NativeLib.voiceGetPreset()
     fun setVoicePreset(preset: Int) { NativeLib.voiceSetPreset(preset) }
     fun getVoiceCompressor(): Boolean = NativeLib.voiceGetCompressor()