Pass the file's base path through the markdown parsing pipeline to
allow the icon directive to resolve relative asset paths correctly
using an absolute fallback chain.
Refactor icon rendering to use a custom `marked-directive` instead of
a global icon prefix. This allows icons to use a CSS-based fallback
chain (searching up to 10 directory levels for an `assets/` folder),
making icon paths more robust and removing the need to pass path
prefixes through the component tree.