voice playback: miniaudio cursor ground truth + EMA smooth decay

- SoundManager: add currentPcmCursor()/isCurrentPcmPlaying() via ma_sound_get_cursor_in_pcm_frames / ma_sound_is_playing
- messagedelegate: replace wall-clock position tracking with real miniaudio cursor, remove 200ms hardcoded delay
- messagedelegate: end-of-playback detection via isCurrentPcmPlaying() instead of wall-clock comparison (was killing audio ~200ms early)
- messagedelegate: EMA smoothing of displayFrac/displayElapsed (0.15 factor, 16ms timer)
- messagedelegate: timer continues after audio stops to animate EMA decay to 0 (not mouse-move dependent)
- media_delivery: download progress callback, Ed25519 sig verify, author fallback, content hash check
- conn_mgr: already-connected returns OK with callback instead of error
- chatgui: voice download-pending auto-play, file download progress bar, media bar colors

tools/chatgui/src/mainwindow.cpp

@@ -257,12 +257,15 @@ void MainWindow::setupBridgeCallbacks() {
         }
     });
     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();
+        s_mainWindow->m_messageList->onAttachmentDownloaded(chId, msg_id);
+    });
+    gui_bridge_set_download_progress_cb([](const char* ch_id, int ch_id_len, int64_t msg_id, int blocks_done, int num_blocks) {
+        if (!s_mainWindow) return;
+        QString chId = QString::fromUtf8(ch_id, ch_id_len);
+        s_mainWindow->m_messageList->updateMessageProgress(chId, msg_id, blocks_done, num_blocks);
     });
 }