Audit log: rename, clickable items, snippet, and filter fix

Audit & Governance: renamed Change history to Audit log (route is now /app/settings/governance/audit-log); clicking an entry now links directly to the affected memory, insight, or file; untitled memories show a plain-text content snippet instead of "an item"; the "via Claude" provenance tag no longer clips on long actor emails; and the Event filter dropdown is now solid and readable.