MDViewer mdviewer-v1.4.0

MDViewer V1.4.0: link handler, nav history, macOS codesign

- Unified link handler (event delegation): external URLs → browser,
  anchors → smooth scroll, relative .md → open in-app, others → system app
- will-navigate/setWindowOpenHandler guard prevents blank screen on unhandled links
- Navigation history (link-only): back/forward via mouse side buttons or Alt+Left/Right,
  scroll position preserved per entry
- macOS CI: ad-hoc codesign + ditto repack for Apple Silicon compatibility
- artifactName includes ${arch} to prevent x64/arm64 zip overwrite

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