feat: add advanced audio effects and improve UI

Phase 6.5 Advanced Effects:
- Add Pitch Shifter with semitones and cents adjustment
- Add Time Stretch with pitch preservation using overlap-add
- Add Distortion with soft/hard/tube types and tone control
- Add Bitcrusher with bit depth and sample rate reduction
- Add AdvancedParameterDialog with real-time waveform visualization
- Add 4 professional presets per effect type

Improvements:
- Fix undefined parameter errors by adding nullish coalescing operators
- Add global custom scrollbar styling with color-mix transparency
- Add custom-scrollbar utility class for side panel
- Improve theme-aware scrollbar appearance in light/dark modes
- Fix parameter initialization when switching effect types

Integration:
- All advanced effects support undo/redo via EffectCommand
- Effects accessible via command palette and side panel
- Selection-based processing support
- Toast notifications for all effects

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/audio/effects/reverse.ts

@@ -0,0 +1,31 @@
+/**
+ * Reverse audio effect
+ */
+
+import { getAudioContext } from '../context';
+
+/**
+ * Reverse audio buffer
+ * @param buffer - Source audio buffer
+ * @returns New audio buffer with reversed audio
+ */
+export function reverseAudio(buffer: AudioBuffer): AudioBuffer {
+  const audioContext = getAudioContext();
+  const outputBuffer = audioContext.createBuffer(
+    buffer.numberOfChannels,
+    buffer.length,
+    buffer.sampleRate
+  );
+
+  // Reverse each channel
+  for (let channel = 0; channel < buffer.numberOfChannels; channel++) {
+    const inputData = buffer.getChannelData(channel);
+    const outputData = outputBuffer.getChannelData(channel);
+
+    for (let i = 0; i < buffer.length; i++) {
+      outputData[i] = inputData[buffer.length - 1 - i];
+    }
+  }
+
+  return outputBuffer;
+}