feat: add presets, Randomize button, menu highlight, finer LFO depth, preset-switch fade-out

- PresetManager: add ~40 new presets including a new Techno category with
  LFO-to-OSC-pitch movement, plus Bass/Lead/Pad/Keys/Brass/Strings/Wind/
  Arp/Pluck/FX expansions
- UI: add RANDOM button; highlight the currently selected entry in all
  dropdown menus (incl. ticking the active preset in the preset browser)
- LFO depth: finer 0.001 step with a skew curve so the lower range resolves
  much finer than the upper range (shared by LFO1 and LFO2)
- DSP: fade out active voices over 250ms on preset change / randomize so long
  release tails from the previous sound are silenced
- Update bg.png

Source/PluginEditor.h

@@ -156,6 +156,7 @@ private:
     juce::TextButton presetButton{"PRESET"};
     PatchLCD patchLCD;
     juce::TextButton prevPresetButton{"<"}, nextPresetButton{">"};
+    juce::TextButton randomizeButton{"RANDOM"};
     int currentPresetIndex = 0;
 
     juce::ComboBox uiScaleBox;
@@ -201,6 +202,7 @@ private:
                        juce::ComboBox& syncBox);
     void showPresetMenu();
     void applyCurrentPreset();
+    void randomizeParameters();
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR(MainContentComponent)
 };