Base64 encoder

Encodes text as Base64 through UTF-8, so accented characters and emoji work — the naive browser approach throws on both.

Base64 turns three bytes into four printable characters, making it about 33 percent larger than what went in. That is the price of putting binary data somewhere only text is allowed. The URL-safe variant swaps `+` and `/` for `-` and `_` and drops the padding, because the standard alphabet breaks in a URL or a filename. The important thing to be clear about: Base64 is an encoding, not encryption. Anyone can decode it instantly, so encoding a password or a token in Base64 protects nothing at all.