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)

lib/opus_codec.h

@@ -0,0 +1,33 @@
+#ifndef OPUS_CODEC_H
+#define OPUS_CODEC_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct opus_codec_encoder opus_codec_encoder_t;
+typedef struct opus_codec_decoder opus_codec_decoder_t;
+
+opus_codec_encoder_t* opus_codec_encoder_create(int sample_rate, int channels);
+void opus_codec_encoder_destroy(opus_codec_encoder_t* enc);
+int opus_codec_encode(opus_codec_encoder_t* enc, const int16_t* pcm, int frame_samples,
+        uint8_t* out, int out_cap);
+
+opus_codec_decoder_t* opus_codec_decoder_create(int sample_rate, int channels);
+void opus_codec_decoder_destroy(opus_codec_decoder_t* dec);
+int opus_codec_decode(opus_codec_decoder_t* dec, const uint8_t* data, int len,
+        int16_t* pcm, int frame_samples);
+
+int opus_codec_encoder_bitrate_set(opus_codec_encoder_t* enc, int bitrate_bps);
+int opus_codec_encoder_complexity_set(opus_codec_encoder_t* enc, int complexity);
+int opus_codec_encoder_signal_set(opus_codec_encoder_t* enc, int signal_type);
+
+const char* opus_codec_strerror(int err);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif