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/PluginProcessor.h

@@ -71,6 +71,10 @@ public:
 
     int getTransposeSemitones() const;
 
+    // Fade out all currently sounding voices over `seconds` so a long release
+    // tail from the previous preset is silenced on preset change.
+    void fadeOutActiveVoices(float seconds);
+
     std::atomic<float> pitchBendValue{0.0f};
     float getPitchBend() const { return pitchBendValue.load(std::memory_order_relaxed); }