feat: initial fft spectrum visualization support

The final goal is actually still clone ShadowPlayer's spectrum
visualization, tho.

fftspectrum.h

@@ -0,0 +1,31 @@
+// SPDX-FileCopyrightText: 2024 Gary Wang <git@blumia.net>
+//
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <QWidget>
+#include <QMediaPlayer>
+
+class QAudioBufferOutput;
+class FFTSpectrum : public QWidget
+{
+    Q_OBJECT
+    Q_PROPERTY(QMediaPlayer * mediaPlayer MEMBER m_mediaPlayer WRITE setMediaPlayer NOTIFY mediaPlayerChanged)
+public:
+    explicit FFTSpectrum(QWidget* parent);
+    ~FFTSpectrum();
+
+    void setMediaPlayer(QMediaPlayer* player);
+
+signals:
+    void mediaPlayerChanged();
+
+protected:
+    void paintEvent(QPaintEvent* e) override;
+
+private:
+    QMediaPlayer* m_mediaPlayer = nullptr;
+    QAudioBufferOutput* m_audioBufferOutput = nullptr;
+    std::vector<float> m_freq;
+};