feat: seeking support and Last.fm scrobbling

Seeking:
- Rust player: seek_requested/seek_target_secs atomics on PlayerStatus
- Decoder loop checks for seek each iteration, calls format.seek() and resets decoder
- New qobuz_backend_seek C FFI + QobuzBackend::seek(quint64)
- Progress slider onProgressReleased now seeks to the dragged position

Last.fm:
- LastFmScrobbler: now-playing + scrobble (50% or 240s threshold, min 30s)
- API signature follows Last.fm spec (sorted params, md5)
- Settings dialog: API key/secret, username/password, Connect button with status
- AppSettings: lastfm/enabled, api_key, api_secret, session_key
- Scrobbler wired to trackChanged, positionChanged, trackFinished in MainWindow

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/dialog/settings.cpp

@@ -1,15 +1,20 @@
 #include "settings.hpp"
 #include "../util/settings.hpp"
+#include "../scrobbler/lastfm.hpp"
 
 #include <QVBoxLayout>
 #include <QFormLayout>
 #include <QDialogButtonBox>
 #include <QGroupBox>
+#include <QHBoxLayout>
+#include <QNetworkReply>
 
 SettingsDialog::SettingsDialog(QWidget *parent) : QDialog(parent)
 {
     setWindowTitle(tr("Settings"));
-    setMinimumWidth(400);
+    setMinimumWidth(420);
+
+    m_nam = new QNetworkAccessManager(this);
 
     auto *layout = new QVBoxLayout(this);
 
@@ -30,25 +35,98 @@ SettingsDialog::SettingsDialog(QWidget *parent) : QDialog(parent)
             break;
         }
     }
-
     playLayout->addRow(tr("Preferred quality:"), m_formatBox);
     layout->addWidget(playGroup);
 
+    // --- Last.fm group ---
+    auto *lfmGroup  = new QGroupBox(tr("Last.fm Scrobbling"), this);
+    auto *lfmLayout = new QFormLayout(lfmGroup);
+
+    m_lastFmEnabled = new QCheckBox(tr("Enable scrobbling"), lfmGroup);
+    m_lastFmEnabled->setChecked(AppSettings::instance().lastFmEnabled());
+    lfmLayout->addRow(m_lastFmEnabled);
+
+    m_lastFmApiKey = new QLineEdit(AppSettings::instance().lastFmApiKey(), lfmGroup);
+    m_lastFmApiKey->setPlaceholderText(tr("Get one at last.fm/api"));
+    lfmLayout->addRow(tr("API Key:"), m_lastFmApiKey);
+
+    m_lastFmApiSecret = new QLineEdit(AppSettings::instance().lastFmApiSecret(), lfmGroup);
+    m_lastFmApiSecret->setEchoMode(QLineEdit::Password);
+    lfmLayout->addRow(tr("API Secret:"), m_lastFmApiSecret);
+
+    m_lastFmUsername = new QLineEdit(lfmGroup);
+    m_lastFmUsername->setPlaceholderText(tr("Last.fm username"));
+    lfmLayout->addRow(tr("Username:"), m_lastFmUsername);
+
+    m_lastFmPassword = new QLineEdit(lfmGroup);
+    m_lastFmPassword->setEchoMode(QLineEdit::Password);
+    m_lastFmPassword->setPlaceholderText(tr("Last.fm password"));
+    lfmLayout->addRow(tr("Password:"), m_lastFmPassword);
+
+    auto *connectRow = new QHBoxLayout;
+    m_lastFmConnect = new QPushButton(tr("Connect"), lfmGroup);
+    m_lastFmStatus  = new QLabel(lfmGroup);
+
+    const bool hasSession = !AppSettings::instance().lastFmSessionKey().isEmpty();
+    m_lastFmStatus->setText(hasSession ? tr("Connected ✓") : tr("Not connected"));
+    m_lastFmStatus->setStyleSheet(hasSession ? "color: green;" : "color: gray;");
+
+    connectRow->addWidget(m_lastFmConnect);
+    connectRow->addWidget(m_lastFmStatus, 1);
+    lfmLayout->addRow(connectRow);
+
+    layout->addWidget(lfmGroup);
     layout->addStretch();
 
     auto *buttons = new QDialogButtonBox(
         QDialogButtonBox::Ok | QDialogButtonBox::Cancel, this);
     layout->addWidget(buttons);
 
-    connect(buttons, &QDialogButtonBox::accepted, this, [this] {
-        applyChanges();
-        accept();
-    });
+    connect(m_lastFmConnect, &QPushButton::clicked, this, &SettingsDialog::onLastFmConnect);
+    connect(buttons, &QDialogButtonBox::accepted, this, [this] { applyChanges(); accept(); });
     connect(buttons, &QDialogButtonBox::rejected, this, &QDialog::reject);
 }
 
 void SettingsDialog::applyChanges()
 {
-    AppSettings::instance().setPreferredFormat(
-        m_formatBox->currentData().toInt());
+    AppSettings::instance().setPreferredFormat(m_formatBox->currentData().toInt());
+    AppSettings::instance().setLastFmEnabled(m_lastFmEnabled->isChecked());
+    AppSettings::instance().setLastFmApiKey(m_lastFmApiKey->text().trimmed());
+    AppSettings::instance().setLastFmApiSecret(m_lastFmApiSecret->text().trimmed());
+}
+
+void SettingsDialog::onLastFmConnect()
+{
+    const QString apiKey    = m_lastFmApiKey->text().trimmed();
+    const QString apiSecret = m_lastFmApiSecret->text().trimmed();
+    const QString username  = m_lastFmUsername->text().trimmed();
+    const QString password  = m_lastFmPassword->text();
+
+    if (apiKey.isEmpty() || apiSecret.isEmpty() || username.isEmpty() || password.isEmpty()) {
+        m_lastFmStatus->setText(tr("Fill in all fields first."));
+        m_lastFmStatus->setStyleSheet("color: red;");
+        return;
+    }
+
+    // Temporarily apply so the scrobbler util can use them for signing
+    AppSettings::instance().setLastFmApiKey(apiKey);
+    AppSettings::instance().setLastFmApiSecret(apiSecret);
+
+    m_lastFmConnect->setEnabled(false);
+    m_lastFmStatus->setText(tr("Connecting…"));
+    m_lastFmStatus->setStyleSheet("color: gray;");
+
+    // Reuse LastFmScrobbler::authenticate as a utility (temp instance)
+    auto *tmp = new LastFmScrobbler(this);
+    tmp->authenticate(username, password, [this, tmp](bool ok, const QString &err) {
+        tmp->deleteLater();
+        m_lastFmConnect->setEnabled(true);
+        if (ok) {
+            m_lastFmStatus->setText(tr("Connected ✓"));
+            m_lastFmStatus->setStyleSheet("color: green;");
+        } else {
+            m_lastFmStatus->setText(tr("Error: %1").arg(err));
+            m_lastFmStatus->setStyleSheet("color: red;");
+        }
+    });
 }

src/dialog/settings.hpp

@@ -3,6 +3,10 @@
 #include <QDialog>
 #include <QComboBox>
 #include <QLabel>
+#include <QLineEdit>
+#include <QCheckBox>
+#include <QPushButton>
+#include <QNetworkAccessManager>
 
 class SettingsDialog : public QDialog
 {
@@ -12,7 +16,19 @@ public:
     explicit SettingsDialog(QWidget *parent = nullptr);
 
 private:
+    // Playback
     QComboBox *m_formatBox = nullptr;
 
+    // Last.fm
+    QCheckBox  *m_lastFmEnabled    = nullptr;
+    QLineEdit  *m_lastFmApiKey     = nullptr;
+    QLineEdit  *m_lastFmApiSecret  = nullptr;
+    QLineEdit  *m_lastFmUsername   = nullptr;
+    QLineEdit  *m_lastFmPassword   = nullptr;
+    QPushButton *m_lastFmConnect   = nullptr;
+    QLabel      *m_lastFmStatus    = nullptr;
+    QNetworkAccessManager *m_nam   = nullptr;
+
     void applyChanges();
+    void onLastFmConnect();
 };