chatgui: add sound playback via miniaudio (MP3 notification sounds)

- SoundManager singleton wraps ma_engine + ma_sound (non-blocking, background thread)
- MP3 files embedded in QRC, decoded to PCM at startup with MA_SOUND_FLAG_DECODE
- miniaudio.h (MIT) in lib/, IMPLEMENTATION in separate miniaudio_impl.c
- Sound settings page in SettingsDialog: enable/disable, volume slider (0-100%)
- Config keys [gui] sound_enabled, sound_volume
- Incoming messages trigger msg_incoming.mp3 sound

tools/chatgui/src/soundsettingspage.cpp

@@ -0,0 +1,120 @@
+#include "soundsettingspage.h"
+#include "sound_manager.h"
+#include <QVBoxLayout>
+#include <QHBoxLayout>
+#include <QCheckBox>
+#include <QSlider>
+#include <QLabel>
+#include <QFile>
+#include <QTextStream>
+#include <QDebug>
+
+SoundSettingsPage::SoundSettingsPage(QWidget* parent)
+    : QWidget(parent)
+{
+    auto* layout = new QVBoxLayout(this);
+    layout->setContentsMargins(0, 12, 0, 0);
+    layout->setSpacing(14);
+
+    m_enabledCheck = new QCheckBox("Enable sounds", this);
+    m_enabledCheck->setStyleSheet("font-size: 13px;");
+    layout->addWidget(m_enabledCheck);
+
+    auto* volLayout = new QHBoxLayout();
+    auto* volLabel = new QLabel("Volume:", this);
+    volLabel->setStyleSheet("font-size: 13px;");
+    volLayout->addWidget(volLabel);
+
+    m_volumeSlider = new QSlider(Qt::Horizontal, this);
+    m_volumeSlider->setRange(0, 100);
+    m_volumeSlider->setFixedWidth(200);
+    m_volumeSlider->setStyleSheet("QSlider::groove:horizontal { height: 6px; border-radius: 3px; background: palette(mid); }"
+                                   "QSlider::handle:horizontal { width: 14px; height: 14px; margin: -4px 0; border-radius: 7px; background: #3390EC; }");
+    volLayout->addWidget(m_volumeSlider);
+
+    m_volumeLabel = new QLabel("80%", this);
+    m_volumeLabel->setFixedWidth(36);
+    m_volumeLabel->setStyleSheet("font-size: 13px;");
+    volLayout->addWidget(m_volumeLabel);
+    volLayout->addStretch();
+    layout->addLayout(volLayout);
+
+    layout->addStretch();
+
+    connect(m_enabledCheck, &QCheckBox::stateChanged, this, &SoundSettingsPage::onEnabledChanged);
+    connect(m_volumeSlider, &QSlider::valueChanged, this, [this](int val) {
+        m_volumeLabel->setText(QString::number(val) + "%");
+    });
+}
+
+void SoundSettingsPage::onEnabledChanged(int state) {
+    m_volumeSlider->setEnabled(state == Qt::Checked);
+    m_volumeLabel->setEnabled(state == Qt::Checked);
+}
+
+void SoundSettingsPage::loadFromConfig(const QString& path) {
+    QFile f(path);
+    if (!f.open(QIODevice::ReadOnly | QIODevice::Text)) return;
+
+    bool enabled = true;
+    int  volume  = 80;
+    bool inGui = false;
+    QTextStream in(&f);
+    while (!in.atEnd()) {
+        QString line = in.readLine().trimmed();
+        if (line == "[gui]") { inGui = true; continue; }
+        if (line.startsWith("[") && line.endsWith("]")) inGui = false;
+        if (!inGui) continue;
+        if (line.startsWith("sound_enabled="))
+            enabled = line.mid(QString("sound_enabled=").size()).trimmed() != "0";
+        else if (line.startsWith("sound_volume="))
+            volume = line.mid(QString("sound_volume=").size()).trimmed().toInt();
+    }
+    f.close();
+
+    m_enabledCheck->setChecked(enabled);
+    m_volumeSlider->setValue(volume);
+    m_volumeSlider->setEnabled(enabled);
+    m_volumeLabel->setEnabled(enabled);
+    m_volumeLabel->setText(QString::number(volume) + "%");
+}
+
+void SoundSettingsPage::saveToConfig(const QString& path) const {
+    QFile f(path);
+    if (!f.open(QIODevice::ReadOnly | QIODevice::Text)) return;
+    QString content = f.readAll();
+    f.close();
+
+    QStringList lines = content.split('\n');
+
+    bool inGui = false;
+    bool foundEnabled = false, foundVolume = false;
+    for (int i = 0; i < lines.size(); i++) {
+        QString line = lines[i].trimmed();
+        if (line == "[gui]") { inGui = true; continue; }
+        if (line.startsWith("[") && line.endsWith("]")) inGui = false;
+        if (!inGui) continue;
+
+        if (line.startsWith("sound_enabled=")) {
+            lines[i] = QString("sound_enabled=%1").arg(m_enabledCheck->isChecked() ? 1 : 0);
+            foundEnabled = true;
+        } else if (line.startsWith("sound_volume=")) {
+            lines[i] = QString("sound_volume=%1").arg(m_volumeSlider->value());
+            foundVolume = true;
+        }
+    }
+
+    if (!foundEnabled || !foundVolume) {
+        for (int i = 0; i < lines.size(); i++) {
+            if (lines[i].trimmed() == "[gui]") {
+                if (!foundEnabled) lines.insert(i + 1, QString("sound_enabled=%1").arg(m_enabledCheck->isChecked() ? 1 : 0));
+                if (!foundVolume) lines.insert(i + 1, QString("sound_volume=%1").arg(m_volumeSlider->value()));
+                break;
+            }
+        }
+    }
+
+    f.open(QIODevice::WriteOnly | QIODevice::Truncate | QIODevice::Text);
+    f.write(lines.join('\n').toUtf8());
+    f.close();
+}