Add reusable base64 encoding/decoding utility functions for handling
binary data and string conversions in deeplink imports.
Features:
- encodeBase64: Encode string to base64
- decodeBase64: Decode base64 to string
- Uses browser-native btoa/atob with proper UTF-8 handling
This utility will be used for encoding prompt content and configuration
files in deeplink URLs.
YoVinchen