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/gain.ts

@@ -0,0 +1,52 @@
+/**
+ * Gain/Volume adjustment effect
+ */
+
+import { getAudioContext } from '../context';
+
+/**
+ * Apply gain to an audio buffer
+ * @param buffer - Source audio buffer
+ * @param gainValue - Gain multiplier (1.0 = no change, 0.5 = -6dB, 2.0 = +6dB)
+ * @returns New audio buffer with gain applied
+ */
+export function applyGain(buffer: AudioBuffer, gainValue: number): AudioBuffer {
+  const audioContext = getAudioContext();
+  const outputBuffer = audioContext.createBuffer(
+    buffer.numberOfChannels,
+    buffer.length,
+    buffer.sampleRate
+  );
+
+  // Apply gain to 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[i] * gainValue;
+      // Clamp to prevent distortion
+      outputData[i] = Math.max(-1, Math.min(1, outputData[i]));
+    }
+  }
+
+  return outputBuffer;
+}
+
+/**
+ * Convert dB to gain multiplier
+ * @param db - Decibels
+ * @returns Gain multiplier
+ */
+export function dbToGain(db: number): number {
+  return Math.pow(10, db / 20);
+}
+
+/**
+ * Convert gain multiplier to dB
+ * @param gain - Gain multiplier
+ * @returns Decibels
+ */
+export function gainToDb(gain: number): number {
+  return 20 * Math.log10(gain);
+}