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/selection.ts

@@ -0,0 +1,128 @@
+/**
+ * Utilities for applying effects to audio selections
+ */
+
+import type { Selection } from '@/types/selection';
+import { getAudioContext } from '../context';
+
+/**
+ * Extract a region from an audio buffer
+ */
+export function extractRegion(
+  buffer: AudioBuffer,
+  startTime: number,
+  endTime: number
+): AudioBuffer {
+  const audioContext = getAudioContext();
+  const sampleRate = buffer.sampleRate;
+  const numberOfChannels = buffer.numberOfChannels;
+
+  const startSample = Math.floor(startTime * sampleRate);
+  const endSample = Math.floor(endTime * sampleRate);
+  const length = endSample - startSample;
+
+  const regionBuffer = audioContext.createBuffer(
+    numberOfChannels,
+    length,
+    sampleRate
+  );
+
+  for (let channel = 0; channel < numberOfChannels; channel++) {
+    const sourceData = buffer.getChannelData(channel);
+    const targetData = regionBuffer.getChannelData(channel);
+
+    for (let i = 0; i < length; i++) {
+      targetData[i] = sourceData[startSample + i];
+    }
+  }
+
+  return regionBuffer;
+}
+
+/**
+ * Replace a region in an audio buffer with processed audio
+ */
+export function replaceRegion(
+  originalBuffer: AudioBuffer,
+  processedRegion: AudioBuffer,
+  startTime: number
+): AudioBuffer {
+  const audioContext = getAudioContext();
+  const sampleRate = originalBuffer.sampleRate;
+  const numberOfChannels = originalBuffer.numberOfChannels;
+
+  // Create new buffer with same length as original
+  const newBuffer = audioContext.createBuffer(
+    numberOfChannels,
+    originalBuffer.length,
+    sampleRate
+  );
+
+  const startSample = Math.floor(startTime * sampleRate);
+
+  for (let channel = 0; channel < numberOfChannels; channel++) {
+    const originalData = originalBuffer.getChannelData(channel);
+    const processedData = processedRegion.getChannelData(channel);
+    const newData = newBuffer.getChannelData(channel);
+
+    // Copy everything from original
+    for (let i = 0; i < originalBuffer.length; i++) {
+      newData[i] = originalData[i];
+    }
+
+    // Replace the selected region with processed data
+    for (let i = 0; i < processedRegion.length; i++) {
+      if (startSample + i < newBuffer.length) {
+        newData[startSample + i] = processedData[i];
+      }
+    }
+  }
+
+  return newBuffer;
+}
+
+/**
+ * Apply an effect function to a selection, or entire buffer if no selection
+ */
+export function applyEffectToSelection(
+  buffer: AudioBuffer,
+  selection: Selection | null,
+  effectFn: (buffer: AudioBuffer) => AudioBuffer
+): AudioBuffer {
+  if (!selection || selection.start === selection.end) {
+    // No selection, apply to entire buffer
+    return effectFn(buffer);
+  }
+
+  // Extract the selected region
+  const region = extractRegion(buffer, selection.start, selection.end);
+
+  // Apply effect to the region
+  const processedRegion = effectFn(region);
+
+  // Replace the region in the original buffer
+  return replaceRegion(buffer, processedRegion, selection.start);
+}
+
+/**
+ * Apply an async effect function to a selection, or entire buffer if no selection
+ */
+export async function applyAsyncEffectToSelection(
+  buffer: AudioBuffer,
+  selection: Selection | null,
+  effectFn: (buffer: AudioBuffer) => Promise<AudioBuffer>
+): Promise<AudioBuffer> {
+  if (!selection || selection.start === selection.end) {
+    // No selection, apply to entire buffer
+    return await effectFn(buffer);
+  }
+
+  // Extract the selected region
+  const region = extractRegion(buffer, selection.start, selection.end);
+
+  // Apply effect to the region
+  const processedRegion = await effectFn(region);
+
+  // Replace the region in the original buffer
+  return replaceRegion(buffer, processedRegion, selection.start);
+}