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/DSP/Voice.h

@@ -57,6 +57,9 @@ public:
 
         env.noteOn();
         filterEnv.noteOn();
+
+        quickFade = 1.0f;
+        quickFadeStep = 0.0f;
     }
 
     void stopNote(float velocity, bool allowTailOff) override {
@@ -80,6 +83,10 @@ public:
                 return;
             }
 
+            // Per-voice quick fade (triggered on preset change)
+            if (quickFade < 1.0f)
+                quickFade = std::max(0.0f, quickFade - quickFadeStep);
+
             // LFO outputs
             float lfo1Out = lfo1.getNextSample();
             float lfo2Out = lfo2.getNextSample();
@@ -135,8 +142,13 @@ public:
             left = std::tanh(left * drive) / std::tanh(drive);
             right = std::tanh(right * drive) / std::tanh(drive);
 
-            outputBuffer.addSample(0, i, left * outputLevel);
-            outputBuffer.addSample(1, i, right * outputLevel);
+            outputBuffer.addSample(0, i, left * outputLevel * quickFade);
+            outputBuffer.addSample(1, i, right * outputLevel * quickFade);
+
+            if (quickFade <= 0.0f) {
+                clearCurrentNote();
+                return;
+            }
         }
     }
 
@@ -197,6 +209,16 @@ public:
 
     void setPitchBend(float semitones) { pitchBendSemitones = semitones; }
 
+    // Quickly fade out this voice (e.g. when a preset is switched) so any
+    // long release tail from the previous sound is silenced over `seconds`.
+    void triggerQuickRelease(float seconds) {
+        double sr = getSampleRate();
+        if (sr <= 0.0) sr = 44100.0;
+        quickFadeStep = 1.0f / static_cast<float>(seconds * sr);
+        env.noteOff();
+        filterEnv.noteOff();
+    }
+
 private:
     Oscillator osc1, osc2;
     Filter filter;
@@ -228,4 +250,6 @@ private:
     float pitchBendSemitones = 0.0f;
     int currentMidiNote = 60;
     float noteVelocity = 1.0f;
+    float quickFade = 1.0f;
+    float quickFadeStep = 0.0f;
 };