feat: complete Phase 11.4 - comprehensive audio file import

Implemented advanced audio import capabilities:

**Import Features:**
- Support for WAV, MP3, OGG, FLAC, M4A, AIFF formats
- Sample rate conversion using OfflineAudioContext
- Stereo to mono conversion (equal channel mixing)
- Normalize on import option (99% peak with 1% headroom)
- Comprehensive codec detection from MIME types and extensions

**API Enhancements:**
- ImportOptions interface (convertToMono, targetSampleRate, normalizeOnImport)
- importAudioFile() function returning buffer + metadata
- AudioFileInfo with AudioMetadata (codec, duration, channels, sample rate, file size)
- Enhanced decodeAudioFile() with optional import transformations

**UI Components:**
- ImportDialog component with import settings controls
- Sample rate selector (44.1kHz - 192kHz)
- Checkbox options for mono conversion and normalization
- File info display (original sample rate and channels)
- Updated FileUpload to show AIFF support

**Technical Implementation:**
- Offline resampling for quality preservation
- Equal-power channel mixing for stereo-to-mono
- Peak detection across all channels
- Metadata extraction with codec identification

Phase 11 (Export & Import) now complete!

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

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

components/editor/FileUpload.tsx

@@ -84,7 +84,7 @@ export function FileUpload({ onFileSelect, className }: FileUploadProps) {
             Click to browse or drag and drop
           </p>
           <p className="text-xs text-muted-foreground mt-2">
-            Supported formats: WAV, MP3, OGG, FLAC, AAC, M4A
+            Supported formats: WAV, MP3, OGG, FLAC, AAC, M4A, AIFF
           </p>
         </div>
       </div>