Add octave offset dropdown, fix waveform sidebar, embed audio in plugin state

- Add octave offset combo (-3 to +3) in title bar for MIDI input transposition
- Fix waveform sidebar base note from C3 to C4
- Fix midiNoteToSliceIndex to compare pitch classes instead of raw MIDI notes
- Add colored slice indicator dots on waveform sidebar keys
- Embed audio data, file path, sample rate, and slices in plugin state for
  DAW project save/restore (falls back to file path if embedded data missing)

Source/PluginEditor.h

@@ -41,6 +41,7 @@ private:
     juce::TextButton zoomOutButton { "-" };
     juce::TextButton zoomResetButton { "1:1" };
     juce::ComboBox scaleComboBox;
+    juce::ComboBox octaveComboBox;
 
     static constexpr int baseWidth = 1100;
     static constexpr int baseHeight = 550;
@@ -77,6 +78,7 @@ private:
     std::unique_ptr<ComboBoxAttachment> stretchBeatsAttachment;
     std::unique_ptr<ComboBoxAttachment> keyShiftAttachment;
     std::unique_ptr<ComboBoxAttachment> scaleAttachment;
+    std::unique_ptr<ComboBoxAttachment> octaveAttachment;
 
     std::unique_ptr<SliderAttachment> bassAttachment, trebleAttachment, sensitivityAttachment;
     std::unique_ptr<SliderAttachment> ampAttackAttachment, ampDecayAttachment, ampSustainAttachment, ampReleaseAttachment;