Audit fixes: key permissions, lock contention, Docker IP filter, doc updates

Security: identity.key written with 0600 permissions (Unix). Docker bridge
IPs (172.17-31.x) filtered from is_shareable_addr to prevent topology
disclosure in relay introductions.

Lock contention: ManifestPush relay and DeleteRecord CDN notify now gather
connections under lock, then send outside lock.

UI: syncBtn null guard prevents crash on hidden element.

Documentation: design.html version badge updated to v0.4.4. Self Last
Encounter threshold corrected from 3h to 4h. Multi-Device Identity section
rewritten for multi-identity-per-device (complete) + multi-device (planned)
+ post merge (planned). MEMORY.md updated to v0.4.4+ status.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/app.js

@@ -3141,7 +3141,7 @@ $('#share-details-btn').addEventListener('click', () => {
     overlay.querySelector('#share-close-btn').addEventListener('click', () => overlay.remove());
     overlay.addEventListener('click', (e) => { if (e.target === overlay) overlay.remove(); });
 });
-syncBtn.addEventListener('click', doSyncAll);
+if (syncBtn) syncBtn.addEventListener('click', doSyncAll);
 if (copyBtn) copyBtn.addEventListener('click', async () => {
     try {
         await navigator.clipboard.writeText(connectString);