feat: wire image uploads into AI assistant chat (vision support)

- Backend: ChatMessageRequest accepts upload_ids, endpoint fetches
  images from S3, base64-encodes them, passes to Claude as multimodal
  content blocks (vision API)
- Backend: add download_file() to storage_service for fetching from S3
- Frontend: handleSend collects completed upload IDs from pendingUploads
  and includes them in the sendChatMessage API call
- Frontend: prefill handler passes upload IDs from dashboard nav state
- Enables paste-screenshot → AI-sees-it flow end-to-end

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/storage_service.py

@@ -67,6 +67,14 @@ async def upload_file(
     return storage_key
 
 
+def download_file(storage_key: str) -> bytes:
+    """Download a file from S3 and return its contents as bytes."""
+    client = _get_client()
+    buf = BytesIO()
+    client.download_fileobj(settings.STORAGE_BUCKET_NAME, storage_key, buf)
+    return buf.getvalue()
+
+
 def get_presigned_url(storage_key: str) -> str:
     """Generate a time-limited presigned URL for downloading a file."""
     client = _get_client()