Function to_url_b64 

fn to_url_b64(b64: &str) -> String

Convert standard base64 to URL-safe base64 without padding.

Reproduces the legacy client’s Utils.fromB64toUrlB64: + → -, / → _, and = padding stripped. The SendView.key is standard base64; the URL fragment must carry the URL-safe form so the bw receive parser (Utils.fromUrlB64ToArray) decodes it correctly.