feat(handshake): shrink the connection exchange; release v5.9.0

The invitation carried the whole session: both public keys, their signatures, a
salt, a challenge and the full SDP. It now carries only what brings up DTLS —
ICE credentials, the certificate fingerprint, candidates, an expiry — plus a
16-byte commitment to the key material. The key material itself moves to the
DataChannel and is checked against that commitment before it is parsed.

Measured on the live site: 2274 characters across 4 animated QR frames became
151 characters in a single frame.

The safety code is now computed over a transcript of both descriptors and both
key blobs, and the HKDF salt is derived from that same transcript instead of
being transmitted. authProof is replaced by one signature over it.

SBQ2_SEND_ENABLED is the single value that reverts new invitations to SB1.

sw.js

@@ -11,7 +11,7 @@ let DYNAMIC_CACHE = 'securebit-pwa-dynamic-v4.7.56';
 // Build stamp — rewritten by scripts/post-build.js on every release so this file's
 // bytes change each deploy. That is what makes the browser detect a new Service Worker,
 // reinstall it, drop stale caches and (via controllerchange) prompt the page to update.
-const SW_BUILD_VERSION = '1786051485967';
+const SW_BUILD_VERSION = '1786054741114';
 
 // Load version from meta.json on install
 async function getAppVersion() {