feat: add multi-track audio import functionality

Implemented comprehensive multi-file audio import system:
- Created ImportTrackDialog component with drag-and-drop and file browser support
- Updated TrackList to integrate import functionality with dedicated buttons
- Added multi-track-demo page to test and demonstrate import features
- Sequential file processing with automatic track naming from filenames
- Error handling for non-audio files

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

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

components/tracks/ImportTrackDialog.tsx

@@ -0,0 +1,145 @@
+'use client';
+
+import * as React from 'react';
+import { Upload, Plus } from 'lucide-react';
+import { Modal } from '@/components/ui/Modal';
+import { Button } from '@/components/ui/Button';
+import { decodeAudioFile } from '@/lib/audio/decoder';
+
+export interface ImportTrackDialogProps {
+  open: boolean;
+  onClose: () => void;
+  onImportTrack: (buffer: AudioBuffer, name: string) => void;
+}
+
+export function ImportTrackDialog({
+  open,
+  onClose,
+  onImportTrack,
+}: ImportTrackDialogProps) {
+  const [isDragging, setIsDragging] = React.useState(false);
+  const [isLoading, setIsLoading] = React.useState(false);
+  const fileInputRef = React.useRef<HTMLInputElement>(null);
+
+  const handleFiles = async (files: FileList) => {
+    setIsLoading(true);
+
+    try {
+      // Process files sequentially
+      for (let i = 0; i < files.length; i++) {
+        const file = files[i];
+
+        if (!file.type.startsWith('audio/')) {
+          console.warn(`Skipping non-audio file: ${file.name}`);
+          continue;
+        }
+
+        try {
+          const buffer = await decodeAudioFile(file);
+          const trackName = file.name.replace(/\.[^/.]+$/, ''); // Remove extension
+          onImportTrack(buffer, trackName);
+        } catch (error) {
+          console.error(`Failed to import ${file.name}:`, error);
+        }
+      }
+
+      onClose();
+    } finally {
+      setIsLoading(false);
+    }
+  };
+
+  const handleFileSelect = (e: React.ChangeEvent<HTMLInputElement>) => {
+    const files = e.target.files;
+    if (files && files.length > 0) {
+      handleFiles(files);
+    }
+    // Reset input
+    e.target.value = '';
+  };
+
+  const handleDragOver = (e: React.DragEvent) => {
+    e.preventDefault();
+    setIsDragging(true);
+  };
+
+  const handleDragLeave = (e: React.DragEvent) => {
+    e.preventDefault();
+    setIsDragging(false);
+  };
+
+  const handleDrop = (e: React.DragEvent) => {
+    e.preventDefault();
+    setIsDragging(false);
+
+    const files = e.dataTransfer.files;
+    if (files && files.length > 0) {
+      handleFiles(files);
+    }
+  };
+
+  return (
+    <Modal
+      open={open}
+      onClose={onClose}
+      title="Import Audio as Tracks"
+      description="Import one or more audio files as new tracks"
+    >
+      <div className="space-y-4">
+        {/* Drag and Drop Area */}
+        <div
+          onDragOver={handleDragOver}
+          onDragLeave={handleDragLeave}
+          onDrop={handleDrop}
+          className={`
+            border-2 border-dashed rounded-lg p-8 text-center transition-colors
+            ${isDragging
+              ? 'border-primary bg-primary/10'
+              : 'border-border hover:border-primary/50'
+            }
+            ${isLoading ? 'opacity-50 pointer-events-none' : ''}
+          `}
+        >
+          <Upload className="h-12 w-12 mx-auto mb-4 text-muted-foreground" />
+          <p className="text-sm text-foreground mb-2">
+            {isLoading ? 'Importing files...' : 'Drag and drop audio files here'}
+          </p>
+          <p className="text-xs text-muted-foreground mb-4">
+            or
+          </p>
+          <Button
+            onClick={() => fileInputRef.current?.click()}
+            disabled={isLoading}
+          >
+            <Plus className="h-4 w-4 mr-2" />
+            Choose Files
+          </Button>
+          <input
+            ref={fileInputRef}
+            type="file"
+            accept="audio/*"
+            multiple
+            onChange={handleFileSelect}
+            className="hidden"
+          />
+        </div>
+
+        {/* Supported Formats */}
+        <div className="text-xs text-muted-foreground">
+          <p className="font-medium mb-1">Supported formats:</p>
+          <p>MP3, WAV, OGG, FLAC, M4A, AAC, and more</p>
+          <p className="mt-2">
+            💡 Tip: Select multiple files at once or drag multiple files to import them all as separate tracks
+          </p>
+        </div>
+
+        {/* Actions */}
+        <div className="flex justify-end gap-2 pt-2 border-t border-border">
+          <Button variant="outline" onClick={onClose} disabled={isLoading}>
+            Cancel
+          </Button>
+        </div>
+      </div>
+    </Modal>
+  );
+}

components/tracks/TrackList.tsx

@@ -1,9 +1,10 @@
 'use client';
 
 import * as React from 'react';
-import { Plus } from 'lucide-react';
+import { Plus, Upload } from 'lucide-react';
 import { Button } from '@/components/ui/Button';
 import { Track } from './Track';
+import { ImportTrackDialog } from './ImportTrackDialog';
 import type { Track as TrackType } from '@/types/track';
 
 export interface TrackListProps {
@@ -12,6 +13,7 @@ export interface TrackListProps {
   currentTime: number;
   duration: number;
   onAddTrack: () => void;
+  onImportTrack?: (buffer: AudioBuffer, name: string) => void;
   onRemoveTrack: (trackId: string) => void;
   onUpdateTrack: (trackId: string, updates: Partial<TrackType>) => void;
   onSeek?: (time: number) => void;
@@ -23,19 +25,45 @@ export function TrackList({
   currentTime,
   duration,
   onAddTrack,
+  onImportTrack,
   onRemoveTrack,
   onUpdateTrack,
   onSeek,
 }: TrackListProps) {
+  const [importDialogOpen, setImportDialogOpen] = React.useState(false);
+
+  const handleImportTrack = (buffer: AudioBuffer, name: string) => {
+    if (onImportTrack) {
+      onImportTrack(buffer, name);
+    }
+  };
+
   if (tracks.length === 0) {
     return (
-      <div className="flex-1 flex flex-col items-center justify-center gap-4 text-muted-foreground">
-        <p className="text-sm">No tracks yet. Add a track to get started.</p>
-        <Button onClick={onAddTrack} variant="secondary">
-          <Plus className="h-4 w-4 mr-2" />
-          Add Track
-        </Button>
-      </div>
+      <>
+        <div className="flex-1 flex flex-col items-center justify-center gap-4 text-muted-foreground">
+          <p className="text-sm">No tracks yet. Add a track to get started.</p>
+          <div className="flex gap-2">
+            <Button onClick={onAddTrack} variant="secondary">
+              <Plus className="h-4 w-4 mr-2" />
+              Add Empty Track
+            </Button>
+            {onImportTrack && (
+              <Button onClick={() => setImportDialogOpen(true)} variant="secondary">
+                <Upload className="h-4 w-4 mr-2" />
+                Import Audio Files
+              </Button>
+            )}
+          </div>
+        </div>
+        {onImportTrack && (
+          <ImportTrackDialog
+            open={importDialogOpen}
+            onClose={() => setImportDialogOpen(false)}
+            onImportTrack={handleImportTrack}
+          />
+        )}
+      </>
     );
   }
 
@@ -74,13 +102,33 @@ export function TrackList({
         ))}
       </div>
 
-      {/* Add Track Button */}
-      <div className="p-2 border-t border-border bg-card">
+      {/* Add Track Buttons */}
+      <div className="p-2 border-t border-border bg-card space-y-2">
         <Button onClick={onAddTrack} variant="outline" size="sm" className="w-full">
           <Plus className="h-4 w-4 mr-2" />
-          Add Track
+          Add Empty Track
         </Button>
+        {onImportTrack && (
+          <Button
+            onClick={() => setImportDialogOpen(true)}
+            variant="outline"
+            size="sm"
+            className="w-full"
+          >
+            <Upload className="h-4 w-4 mr-2" />
+            Import Audio Files
+          </Button>
+        )}
       </div>
+
+      {/* Import Dialog */}
+      {onImportTrack && (
+        <ImportTrackDialog
+          open={importDialogOpen}
+          onClose={() => setImportDialogOpen(false)}
+          onImportTrack={handleImportTrack}
+        />
+      )}
     </div>
   );
 }