media: рефакторинг загрузки блоков — конечный автомат, фейловер, watchdog простоя

- media_download: состояние блока (IDLE/CONN/REQ/RECV/WAIT/DONE/FAILED),
  кандидаты-держатели (MD_MAX_HOLDERS_PER_BLOCK), фейловер + ретраи (MEDIA_DL_MAX_ATTEMPTS),
  watchdog простоя (MEDIA_DL_STALL_TIMEOUT_TB) вместо фейкового времени
- media_delivery: настраиваемые dl_stall_timeout_tb / dl_max_attempts
- chat_msg: пометка ошибки скачивания в local_attrs ("st":"er") для UI
- tests: новый test_media_download_timeout (watchdog/фейловер), рефакторинг
  test_media_delivery_download/full
- chatgui-android: состояние 'error' у вложений/видео (иконка + tap to retry)

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

@@ -274,6 +274,7 @@ class ChatRepository {
                 val localAttrs = obj.optString("localAttrs", "")
                 if (filePath.isNotEmpty()) downloadState = "downloaded"
                 else if (localAttrs.contains("\"st\":\"fl\"")) downloadState = "downloaded"
+                else if (localAttrs.contains("\"st\":\"er\"")) downloadState = "error"
 
                 var waveform = emptyList<Float>()
                 var voiceDurationMs = 0

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

@@ -208,11 +208,13 @@ private fun FileBubble(message: Message, textColor: Color, onDownload: (Message)
     val stateIcon = when {
         state == "downloaded" -> "\u2705"
         state == "downloading" -> "\u23F3"
+        state == "error" -> "\u26A0\uFE0F"
         else -> "\u2B07\uFE0F"
     }
     val stateText = when {
         state == "downloaded" -> "Tap to save"
         state == "downloading" -> "Downloading..."
+        state == "error" -> "Download failed, tap to retry"
         else -> "Tap to download"
     }
 
@@ -220,7 +222,7 @@ private fun FileBubble(message: Message, textColor: Color, onDownload: (Message)
         modifier = Modifier.clickable {
             when {
                 state == "downloaded" -> onOpen(message)
-                state.isEmpty() -> onDownload(message)
+                else -> onDownload(message)
             }
         }
     ) {
@@ -272,6 +274,7 @@ private fun VideoBubble(message: Message, textColor: Color,
                         onPlay: (Message) -> Unit = {}) {
     val downloading = message.downloadState == "downloading"
     val downloaded = message.filePath.isNotEmpty()
+    val isError = message.downloadState == "error"
 
     var thumb by remember(message.filePath) { mutableStateOf<Bitmap?>(null) }
     var durMs by remember(message.filePath) { mutableStateOf(0) }
@@ -302,7 +305,7 @@ private fun VideoBubble(message: Message, textColor: Color,
             Box(modifier = Modifier.fillMaxSize(), contentAlignment = Alignment.Center) {
                 Box(modifier = Modifier.size(44.dp).background(Color(0x88000000), CircleShape), contentAlignment = Alignment.Center) {
                     Text(
-                        when { downloading -> "\u23F3"; !downloaded -> "\u2B07"; else -> "\u25B6" },
+                        when { downloading -> "\u23F3"; isError -> "\u267B"; !downloaded -> "\u2B07"; else -> "\u25B6" },
                         fontSize = 18.sp, color = Color.White
                     )
                 }
@@ -323,6 +326,7 @@ private fun VideoBubble(message: Message, textColor: Color,
         Text(
             "${formatFileSize(message.fileSize)} · " + when {
                 downloading -> "Downloading..."
+                isError -> "Failed, tap to retry"
                 !downloaded -> "Tap to download"
                 else -> "Tap to play"
             },