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/src/mainwindow.cpp

@@ -256,6 +256,14 @@ void MainWindow::setupBridgeCallbacks() {
             s_mainWindow->m_messageList->loadChannel(lastCid);
         }
     });
+    gui_bridge_set_attachment_downloaded_cb([](const char* ch_id, int ch_id_len, int64_t msg_id) {
+        (void)msg_id;
+        if (!s_mainWindow) return;
+        QString chId = QString::fromUtf8(ch_id, ch_id_len);
+        s_mainWindow->m_channelList->loadChannels();
+        if (chId == s_mainWindow->m_currentChannelId)
+            s_mainWindow->m_messageList->refresh();
+    });
 }
 
 void MainWindow::setupConnects() {