feat(web): preview Markdown run files

[fkukuck]

Summary

• render changed .md run files as Markdown by default
• add a Preview/Changes toggle so reviewers can switch back to the diff
• promote the existing guarded Markdown renderer to a shared component instead of introducing a parallel rendering dependency
• retain raw-HTML stripping and unsafe-URL filtering, with focused tests

Testing

• cd apps/fabro-web && bun test --isolate (800 passed)
• cd apps/fabro-web && bun run typecheck
• cd apps/fabro-web && bun run build

This PR is opened in fkukuck/fabro for review before proposing the change upstream.