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/fade.ts

@@ -0,0 +1,116 @@
+/**
+ * Fade in/out effects
+ */
+
+import { getAudioContext } from '../context';
+
+export type FadeType = 'linear' | 'exponential' | 'logarithmic';
+
+/**
+ * Apply fade in to audio buffer
+ * @param buffer - Source audio buffer
+ * @param duration - Fade duration in seconds
+ * @param type - Fade curve type
+ * @returns New audio buffer with fade in applied
+ */
+export function applyFadeIn(
+  buffer: AudioBuffer,
+  duration: number,
+  type: FadeType = 'linear'
+): AudioBuffer {
+  const audioContext = getAudioContext();
+  const fadeSamples = Math.min(
+    Math.floor(duration * buffer.sampleRate),
+    buffer.length
+  );
+
+  const outputBuffer = audioContext.createBuffer(
+    buffer.numberOfChannels,
+    buffer.length,
+    buffer.sampleRate
+  );
+
+  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++) {
+      if (i < fadeSamples) {
+        const progress = i / fadeSamples;
+        const gain = calculateFadeGain(progress, type);
+        outputData[i] = inputData[i] * gain;
+      } else {
+        outputData[i] = inputData[i];
+      }
+    }
+  }
+
+  return outputBuffer;
+}
+
+/**
+ * Apply fade out to audio buffer
+ * @param buffer - Source audio buffer
+ * @param duration - Fade duration in seconds
+ * @param type - Fade curve type
+ * @returns New audio buffer with fade out applied
+ */
+export function applyFadeOut(
+  buffer: AudioBuffer,
+  duration: number,
+  type: FadeType = 'linear'
+): AudioBuffer {
+  const audioContext = getAudioContext();
+  const fadeSamples = Math.min(
+    Math.floor(duration * buffer.sampleRate),
+    buffer.length
+  );
+  const fadeStartSample = buffer.length - fadeSamples;
+
+  const outputBuffer = audioContext.createBuffer(
+    buffer.numberOfChannels,
+    buffer.length,
+    buffer.sampleRate
+  );
+
+  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++) {
+      if (i >= fadeStartSample) {
+        const progress = (i - fadeStartSample) / fadeSamples;
+        const gain = calculateFadeGain(1 - progress, type);
+        outputData[i] = inputData[i] * gain;
+      } else {
+        outputData[i] = inputData[i];
+      }
+    }
+  }
+
+  return outputBuffer;
+}
+
+/**
+ * Calculate fade gain based on progress and curve type
+ * @param progress - Progress from 0 to 1
+ * @param type - Fade curve type
+ * @returns Gain value from 0 to 1
+ */
+function calculateFadeGain(progress: number, type: FadeType): number {
+  switch (type) {
+    case 'linear':
+      return progress;
+
+    case 'exponential':
+      // Exponential curve: faster at the start, slower at the end
+      return progress * progress;
+
+    case 'logarithmic':
+      // Logarithmic curve: slower at the start, faster at the end
+      return Math.sqrt(progress);
+
+    default:
+      return progress;
+  }
+}