feat: integrate Opus codec into lib/libopus + voice messages in chatgui

- Add lib/libopus/ (opus v1.6.1 embedded source: CELT 18 .c, SILK 77 .c, SILK float 28 .c, OPUS core 14 .c)
- Add lib/opus_codec.h/.c (wrapper: encoder, decoder, raw PCM int16)
- Add tests/test_opus_codec.c (7 tests: silence, sine, compression, PLC, null, defaults, multi-frame)
- chatgui: AudioRecorder (miniaudio capture, 48kHz mono)
- chatgui: VoiceEncoder (PCM→Opus file with custom container)
- chatgui: VoicePlayer (Opus→PCM playback via SoundManager)
- chatgui: PTT button in InputBar, voice message send/receive in MessageList
- chatgui: AudioDeviceSettingsPage (device selection, persisted in ui_state)
- chatgui: MessageDelegate voice message rendering (play button + duration)
- Update autotools: configure.ac, Makefile.am, src/Makefile.am, tests/Makefile.am
- Update CMake: chatgui CMakeLists.txt, libutun CMakeLists.txt (link libopus_internal.a)

tools/chatgui/src/settingsdialog.cpp

@@ -5,6 +5,7 @@
 #include "statuspage.h"
 #include "nodespage.h"
 #include "soundsettingspage.h"
+#include "audiodevicesettingspage.h"
 #include "../transport/gui_bridge.h"
 #include "../../lib/mem.h"
 #include "../db/db_manager.h"
@@ -48,6 +49,7 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, UtunNod
     m_categoryList->addItem("Status");
     m_categoryList->addItem("Nodes");
     m_categoryList->addItem("Sound");
+    m_categoryList->addItem("Audio");
     mainLayout->addWidget(m_categoryList);
 
     // --- Right panel: stacked pages ---
@@ -99,6 +101,9 @@ SettingsDialog::SettingsDialog(const QString& configPath, DbManager* db, UtunNod
 
     m_soundPage = new SoundSettingsPage(db, m_pages);
     m_pages->addWidget(m_soundPage);
+
+    m_audioDevicePage = new AudioDeviceSettingsPage(db, m_pages);
+    m_pages->addWidget(m_audioDevicePage);
     rightLayout->addWidget(m_pages, 1);
 
     // --- Bottom buttons ---
@@ -164,6 +169,7 @@ void SettingsDialog::onCategoryChanged(int row) {
     if (row == 3) m_statusPage->refreshStatus();
     if (row == 4) m_nodesPage->refreshNodes();
     if (row == 5) m_soundPage->loadFromDb();
+    if (row == 6) m_audioDevicePage->loadFromDb();
 }
 
 void SettingsDialog::onSave() {
@@ -219,5 +225,6 @@ void SettingsDialog::onSave() {
     }
 
     m_soundPage->applyAndSave();
+    m_audioDevicePage->applyAndSave();
     accept();
 }