feat: album/playlist header and playlist ownership filtering

- Add TrackContextHeader widget: shows album art (fetched via NAM),
  title, subtitle (artist/description), and metadata (year · tracks · duration)
  above the track list when an album or playlist is opened
- Hide header for favorite tracks and search results
- Store user ID in AppSettings on login
- Only show "Delete playlist" for playlists the user owns
- "Add to playlist" submenu only lists owned playlists
- "Remove from this playlist" only appears when viewing an owned playlist

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

src/mainwindow.cpp

@@ -242,6 +242,9 @@ void MainWindow::onLoginSuccess(const QString &token, const QJsonObject &user)
     const QString email       = user["email"].toString();
     AppSettings::instance().setDisplayName(displayName);
     AppSettings::instance().setUserEmail(email);
+    const qint64 userId = static_cast<qint64>(user["id"].toDouble());
+    if (userId > 0)
+        AppSettings::instance().setUserId(userId);
     statusBar()->showMessage(tr("Signed in as %1").arg(
         displayName.isEmpty() ? email : displayName));
     m_library->refresh();