feat: comprehensive audit log with SQLite persistence and Audit Log view

[DorianZheng]

Summary

• Adds audit_events SQLite table (via better-sqlite3) that persists across restarts
• Hooks auditDb.record() into all workflow mutations (items, tasks, work products)
• Exposes dune:audit:query and dune:audit:export-csv IPC channels
• New Audit Log view with filters (date range, event type, actor, item) and CSV export
• Unit tests for AuditDatabase

Test plan

• pnpm vitest run passes
• pnpm tsc --noEmit passes
• Launch app, perform actions, verify events in Audit Log view
• Test filters and CSV export