refactor: export/import projects as ZIP files instead of JSON

Changed from single JSON file to ZIP archive format to avoid string
length limits when serializing large audio buffers.

New ZIP structure:
- project.json (metadata, track info, effects, automation)
- track_0.wav, track_1.wav, etc. (audio files in WAV format)

Benefits:
- No more RangeError: Invalid string length
- Smaller file sizes (WAV compression vs base64 JSON)
- Human-readable audio files in standard format
- Can extract and inspect individual tracks
- Easier to edit/debug project metadata

Added jszip dependency for ZIP file handling.
Changed file picker to accept .zip files.

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

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

package.json

@@ -11,6 +11,7 @@
   "dependencies": {
     "clsx": "^2.1.1",
     "fflate": "^0.8.2",
+    "jszip": "^3.10.1",
     "lamejs": "github:zhuker/lamejs",
     "lucide-react": "^0.553.0",
     "next": "^16.0.0",