chore(release): renumber group calls as v6.7.0

Group calls are a new capability, and 6.6.6 only landed as a patch because the
version was bumped once per deploy while the feature was being tested. The
number is the release's, not the deploy's.

No code changes: version, badge, changelog heading, docs and build stamp.

Claude-Session: https://claude.ai/code/session_01XSxAkET3hQTkYDQfbjCQwZ

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 = '1788237895492';
+const SW_BUILD_VERSION = '1788239352576';
 
 // Locale subdirectories, rewritten by scripts/build-i18n.js. Each localized page is a
 // separate document at its own URL, so the shell has to be cached and served per