feat: implement multi-track waveform selection and editing with undo/redo

Added comprehensive selection and editing capabilities to multi-track editor:
- Visual selection overlay with Shift+drag interaction on waveforms
- Multi-track edit commands (cut, copy, paste, delete, duplicate)
- Full keyboard shortcut support (Ctrl+X/C/V/D, Delete, Ctrl+Z/Y)
- Complete undo/redo integration via command pattern
- Per-track selection state with localStorage persistence
- Audio buffer manipulation utilities (extract, insert, delete, duplicate segments)
- Toast notifications for all edit operations
- Red playhead to distinguish from blue selection overlay

All edit operations are fully undoable and integrated with the existing
history manager system.

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

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

lib/audio/buffer-utils.ts

@@ -165,3 +165,15 @@ export function concatenateBuffers(
 
   return newBuffer;
 }
+
+/**
+ * Duplicate a segment of audio buffer (extract and insert it after the selection)
+ */
+export function duplicateBufferSegment(
+  buffer: AudioBuffer,
+  startTime: number,
+  endTime: number
+): AudioBuffer {
+  const segment = extractBufferSegment(buffer, startTime, endTime);
+  return insertBufferSegment(buffer, segment, endTime);
+}

lib/audio/track-utils.ts

@@ -37,6 +37,7 @@ export function createTrack(name?: string, color?: TrackColor): Track {
     effectChain: createEffectChain(`${trackName} Effects`),
     collapsed: false,
     selected: false,
+    selection: null,
   };
 }
 

lib/history/commands/multi-track-edit-command.ts

@@ -0,0 +1,190 @@
+/**
+ * Multi-track edit commands for audio operations across tracks
+ */
+
+import { BaseCommand } from '../command';
+import type { Track } from '@/types/track';
+import type { Selection } from '@/types/selection';
+import {
+  extractBufferSegment,
+  deleteBufferSegment,
+  insertBufferSegment,
+  duplicateBufferSegment,
+} from '@/lib/audio/buffer-utils';
+
+export type MultiTrackEditType = 'cut' | 'copy' | 'delete' | 'paste' | 'duplicate';
+
+export interface MultiTrackEditParams {
+  type: MultiTrackEditType;
+  trackId: string;
+  beforeBuffer: AudioBuffer | null;
+  afterBuffer: AudioBuffer | null;
+  selection?: Selection;
+  clipboardData?: AudioBuffer;
+  pastePosition?: number;
+  onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void;
+}
+
+/**
+ * Command for multi-track edit operations
+ */
+export class MultiTrackEditCommand extends BaseCommand {
+  private type: MultiTrackEditType;
+  private trackId: string;
+  private beforeBuffer: AudioBuffer | null;
+  private afterBuffer: AudioBuffer | null;
+  private selection?: Selection;
+  private clipboardData?: AudioBuffer;
+  private pastePosition?: number;
+  private onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void;
+
+  constructor(params: MultiTrackEditParams) {
+    super();
+    this.type = params.type;
+    this.trackId = params.trackId;
+    this.beforeBuffer = params.beforeBuffer;
+    this.afterBuffer = params.afterBuffer;
+    this.selection = params.selection;
+    this.clipboardData = params.clipboardData;
+    this.pastePosition = params.pastePosition;
+    this.onApply = params.onApply;
+  }
+
+  execute(): void {
+    // For copy, don't modify the buffer, just update selection
+    if (this.type === 'copy') {
+      this.onApply(this.trackId, this.beforeBuffer, this.selection || null);
+    } else {
+      this.onApply(this.trackId, this.afterBuffer, null);
+    }
+  }
+
+  undo(): void {
+    this.onApply(this.trackId, this.beforeBuffer, null);
+  }
+
+  getDescription(): string {
+    switch (this.type) {
+      case 'cut':
+        return 'Cut';
+      case 'copy':
+        return 'Copy';
+      case 'delete':
+        return 'Delete';
+      case 'paste':
+        return 'Paste';
+      case 'duplicate':
+        return 'Duplicate';
+      default:
+        return 'Edit';
+    }
+  }
+}
+
+/**
+ * Factory functions to create multi-track edit commands
+ */
+
+export function createMultiTrackCutCommand(
+  trackId: string,
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void
+): MultiTrackEditCommand {
+  const afterBuffer = deleteBufferSegment(buffer, selection.start, selection.end);
+
+  return new MultiTrackEditCommand({
+    type: 'cut',
+    trackId,
+    beforeBuffer: buffer,
+    afterBuffer,
+    selection,
+    onApply,
+  });
+}
+
+export function createMultiTrackCopyCommand(
+  trackId: string,
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void
+): MultiTrackEditCommand {
+  // Copy doesn't modify the buffer
+  return new MultiTrackEditCommand({
+    type: 'copy',
+    trackId,
+    beforeBuffer: buffer,
+    afterBuffer: buffer,
+    selection,
+    onApply,
+  });
+}
+
+export function createMultiTrackDeleteCommand(
+  trackId: string,
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void
+): MultiTrackEditCommand {
+  const afterBuffer = deleteBufferSegment(buffer, selection.start, selection.end);
+
+  return new MultiTrackEditCommand({
+    type: 'delete',
+    trackId,
+    beforeBuffer: buffer,
+    afterBuffer,
+    selection,
+    onApply,
+  });
+}
+
+export function createMultiTrackPasteCommand(
+  trackId: string,
+  buffer: AudioBuffer | null,
+  clipboardData: AudioBuffer,
+  pastePosition: number,
+  onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void
+): MultiTrackEditCommand {
+  const targetBuffer = buffer || createSilentBuffer(clipboardData.sampleRate, clipboardData.numberOfChannels, pastePosition);
+  const afterBuffer = insertBufferSegment(targetBuffer, clipboardData, pastePosition);
+
+  return new MultiTrackEditCommand({
+    type: 'paste',
+    trackId,
+    beforeBuffer: buffer,
+    afterBuffer,
+    clipboardData,
+    pastePosition,
+    onApply,
+  });
+}
+
+export function createMultiTrackDuplicateCommand(
+  trackId: string,
+  buffer: AudioBuffer,
+  selection: Selection,
+  onApply: (trackId: string, buffer: AudioBuffer | null, selection: Selection | null) => void
+): MultiTrackEditCommand {
+  const afterBuffer = duplicateBufferSegment(buffer, selection.start, selection.end);
+
+  return new MultiTrackEditCommand({
+    type: 'duplicate',
+    trackId,
+    beforeBuffer: buffer,
+    afterBuffer,
+    selection,
+    onApply,
+  });
+}
+
+/**
+ * Helper function to create a silent buffer
+ */
+function createSilentBuffer(sampleRate: number, numberOfChannels: number, duration: number): AudioBuffer {
+  const audioContext = new OfflineAudioContext(
+    numberOfChannels,
+    Math.ceil(duration * sampleRate),
+    sampleRate
+  );
+  return audioContext.createBuffer(numberOfChannels, Math.ceil(duration * sampleRate), sampleRate);
+}

lib/hooks/useMultiTrack.ts

@@ -31,6 +31,7 @@ export function useMultiTrack() {
           name: String(t.name || 'Untitled Track'), // Ensure name is always a string
           audioBuffer: null, // Will need to be reloaded
           effectChain: t.effectChain || createEffectChain(`${t.name} Effects`), // Restore effect chain or create new
+          selection: t.selection || null, // Initialize selection
         }));
       }
     } catch (error) {