feat: add image reference validation to lint system

Add new image reference validation rule in manual_lint.js:
- Validates that referenced images exist in the file system
- Supports relative paths (./, ../) and skips external URLs
- Reports missing images as lint errors

Fixes found issue:
- dishes/soup/排骨山药玉米汤/排骨山药玉米汤.md: removed missing image
  reference './排骨山药玉米汤.jpg'

All lint checks now pass ✅

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>