Resolve relative image paths to file:// URLs in rendered HTML

[masawada]

Why

Generated HTML lives under ~/.mdp/, so relative image references like ![diagram](images/a.png) point at a path that does not exist there and show up broken in the browser.

What

• Rewrite the src of every <img> in the rendered HTML to a file:// URL resolved against the markdown file's directory. Values that already have a scheme (https://, data:, etc.) are left as is.
• The rewrite runs on the rendered HTML before the theme template is applied, so it covers both markdown images and raw <img> tags (unsafe: true) without touching the theme.
• Add an e2e case and a README section.