Markdown images and dividers now render at their natural proportions

Two small rendering bugs in the markdown pipeline have been bugging readers for a while. Both are fixed in this release.

Images embedded in markdown content used to render with a forced aspect ratio that cropped or stretched them away from their original shape. Portrait shots got cropped to landscape, screenshots got squeezed. This change drops the forced ratio entirely:

• Images now render at their natural width-to-height ratio, sized to fit the article column.
• Horizontal rules (---) finally got proper styling — a clean centered border instead of the browser's default thin grey line.

If your articles lean on visuals to do storytelling work — diagrams, screenshots, photography, charts — both changes are going to be immediately visible the next time you scroll through a published piece.