Clearer introductions for first-time visitors

People who first encounter Artifact Share through a shared artifact can now learn what the product is without leaving the viewer. Focusing or hovering over the information icon shows a brief product overview. The preview is for viewing only, with no links or buttons. The icon itself links to About: click, press Enter, or tap it to continue. From About, the main call to action leads to Getting Started.

The Home page now offers the same introduction when there are no posts yet. Instead of an empty page with no clear next step, it points people to About and Getting Started so they can understand Artifact Share and choose how to begin.