Add multiple-session support: run several independent encrypted chats at once

Each conversation now runs its own WebRTC session with separate keys and SAS verification, so chats never mix. Adds a side panel to switch between open chats with unread badges, a New chat action that leaves existing chats connected, per-chat local labels stored only on this device, and an availability status (Available, Away, Busy, Invisible) shared end-to-end with connected peers. Also includes vendored Prism syntax highlighting, more reliable PWA update handling, and offline send queueing fixes. Version 4.10.0.

sw.js

@@ -8,6 +8,11 @@ let CACHE_NAME = 'securebit-pwa-v4.7.56';
 let STATIC_CACHE = 'securebit-pwa-static-v4.7.56';
 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 = '1782446255208';
+
 // Load version from meta.json on install
 async function getAppVersion() {
     try {