fix: security hardening and cleanup from community PR cherry-picks

- Add HTML sanitizer for markdown rendering (XSS prevention)
- Switch service worker to network-first caching (deploys take effect immediately)
- Sanitize Content-Disposition filenames (header injection prevention)
- Expose session.muxName getter, replace unsafe `as any` cast
- Static import for execFile, update CLAUDE.md keyboard shortcuts

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