Add persistent UI scale parameter (75%..200%)

- Add uiScale AudioParameterChoice to APVTS with 5 options (75%, 100%, 125%, 150%, 200%)
- Connect uiScaleBox ComboBox to parameter via ComboBoxAttachment
- Restore saved scale on editor creation
- Scale persists across plugin close/reopen via APVTS state

Source/PluginEditor.h

@@ -296,6 +296,7 @@ private:
 
     // Arpeggiator attachments
     std::unique_ptr<ComboBoxAttachment> arpPatternAttach, arpOctavesAttach, arpRateAttach;
+    std::unique_ptr<ComboBoxAttachment> uiScaleAttach;
 
     void setupLabel(juce::Label& label, const juce::String& text);
     void setupKnob(juce::Slider& knob, const juce::String& name);