feat: MP3 download/export capability

Backend:
- New endpoint: GET /api/clips/{id}/download
  - Returns MP3 with Content-Disposition: attachment header
  - Filename derived from clip title (sanitized) or prompt, falls back to clip ID
  - Sanitization: alphanumeric + spaces/hyphens/underscores only, max 60 chars
  - Uses FastAPI FileResponse with media_type=audio/mpeg

Frontend:
- Gallery ribbons: Download button (downward arrow icon) next to delete button
  - Uses window.open() to trigger browser download
  - Hover: lyria-accent color
- Gallery tiles: Download button in tile footer action row
  - Small icon (w-3 h-3), same hover behavior
- DetailPanel: Full-width 'Download MP3' button above Delete Track
  - lyria-accent styled, with download icon
  - Uses <a href> with download attribute for native browser handling

Architecture KB updated with Export/Download section.

backend/src/main.py

@@ -113,6 +113,31 @@ async def stream_audio(clip_id: str):
         raise HTTPException(status_code=404, detail="Audio file not found")
     return FileResponse(file_path, media_type="audio/mpeg", filename=clip["filename"])
 
+@app.get("/api/clips/{clip_id}/download")
+async def download_clip(clip_id: str):
+    clip = await get_clip(clip_id)
+    if not clip:
+        raise HTTPException(status_code=404, detail="Clip not found")
+    file_path = CLIPS_DIR / clip["filename"]
+    if not file_path.exists():
+        raise HTTPException(status_code=404, detail="Audio file not found")
+    
+    # Build download filename from title or prompt
+    base_name = clip.get("title") or clip["prompt"]
+    # Sanitize for filesystem
+    safe_name = "".join(c for c in base_name if c.isalnum() or c in " -_").strip()
+    safe_name = safe_name[:60]  # Limit length
+    if not safe_name:
+        safe_name = clip_id
+    download_filename = f"{safe_name}.mp3"
+    
+    return FileResponse(
+        file_path,
+        media_type="audio/mpeg",
+        filename=download_filename,
+        content_disposition_type="attachment"
+    )
+
 @app.delete("/api/clips/{clip_id}")
 async def remove_clip(clip_id: str):
     clip = await get_clip(clip_id)