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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "securebit-chat",
-  "version": "6.6.6",
+  "version": "6.7.0",
   "description": "Secure P2P Communication Application with End-to-End Encryption",
   "main": "index.html",
   "scripts": {