fix(qr): accept a one-frame invitation; release v5.9.1

The scanner waited for four frames when shown a single one. Its chunk assembler
was written for SB1, which the generator always cuts into exactly four frames,
and its fallback branch claims any non-JSON string longer than 100 characters —
which a 151-character SBQ2 invitation is. A complete invitation was filed as
chunk 1 of 4, and the scan never finished.

SBQ2 payloads are now recognised as complete before any assembly runs, in both
the text and raw-byte forms, and the hard-coded frame count is marked as
belonging to SB1 so it is not read as a general rule.

README.md

@@ -9,7 +9,7 @@
 No accounts. No servers storing your messages. No installation required.
 
 [![License: MIT](https://img.shields.io/badge/License-MIT-f0892a.svg)](LICENSE)
-[![Version](https://img.shields.io/badge/version-5.9.0-3ecf8e.svg)](CHANGELOG.md)
+[![Version](https://img.shields.io/badge/version-5.9.1-3ecf8e.svg)](CHANGELOG.md)
 [![PWA](https://img.shields.io/badge/PWA-installable-3ecf8e.svg)](#install-as-an-app)
 [![Encryption](https://img.shields.io/badge/crypto-ECDH%20P--384%20%C2%B7%20AES--256--GCM-blue.svg)](#security-model)
 [![Forward secrecy](https://img.shields.io/badge/forward%20secrecy-Double%20Ratchet-3ecf8e.svg)](#forward-secrecy)