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/voiceplayback.cpp

@@ -0,0 +1,78 @@
+#include "voiceplayback.h"
+#include "sound_manager.h"
+
+extern "C" {
+#include "../../../lib/opus_codec.h"
+}
+
+#include <QFile>
+#include <QDebug>
+
+static const uint32_t OPUS_MAGIC = 0x5355504F;
+
+int VoicePlayer::decodeOpusFile(const QString& filePath, std::vector<int16_t>& outPcm,
+                                 int& outSampleRate, int& outChannels, float& outDurationSec) {
+    QFile file(filePath);
+    if (!file.open(QIODevice::ReadOnly)) {
+        qWarning("VoicePlayer: cannot open %s", qPrintable(filePath));
+        return -1;
+    }
+
+    uint32_t magic, sr32;
+    uint16_t ch16, frameSamples;
+    if (file.read((char*)&magic, 4) != 4 || magic != OPUS_MAGIC ||
+        file.read((char*)&sr32, 4) != 4 ||
+        file.read((char*)&ch16, 2) != 2 ||
+        file.read((char*)&frameSamples, 2) != 2) {
+        qWarning("VoicePlayer: invalid header in %s", qPrintable(filePath));
+        return -1;
+    }
+
+    int sampleRate = (int)sr32;
+    int channels = (int)ch16;
+    outSampleRate = sampleRate;
+    outChannels = channels;
+
+    opus_codec_decoder_t* dec = opus_codec_decoder_create(sampleRate, channels);
+    if (!dec) {
+        qWarning("VoicePlayer: decoder create failed");
+        return -1;
+    }
+
+    outPcm.clear();
+    int frameCount = 0;
+
+    while (!file.atEnd()) {
+        uint16_t plen;
+        if (file.read((char*)&plen, 2) != 2) break;
+        if (plen == 0) break; /* end marker */
+
+        QByteArray pktData = file.read(plen);
+        if (pktData.size() != (int)plen) break;
+
+        std::vector<int16_t> frameBuf(frameSamples * channels);
+        int decoded = opus_codec_decode(dec, (const uint8_t*)pktData.constData(),
+                                         plen, frameBuf.data(), frameSamples);
+        if (decoded > 0) {
+            size_t cur = outPcm.size();
+            outPcm.resize(cur + decoded * channels);
+            memcpy(outPcm.data() + cur, frameBuf.data(), decoded * channels * sizeof(int16_t));
+            frameCount++;
+        }
+    }
+
+    outDurationSec = (float)frameCount * 20.0f / 1000.0f;
+    qDebug("VoicePlayer: decoded %d frames (%.1fs) from %s", frameCount, outDurationSec, qPrintable(filePath));
+    opus_codec_decoder_destroy(dec);
+    return frameCount;
+}
+
+void VoicePlayer::playOpusFile(const QString& filePath) {
+    std::vector<int16_t> pcm;
+    int sampleRate, channels;
+    float duration;
+
+    if (decodeOpusFile(filePath, pcm, sampleRate, channels, duration) <= 0) return;
+
+    SoundManager::instance()->playRawPcm(pcm.data(), pcm.size() / channels, sampleRate, channels);
+}