feat: redesign fragment detail page with retro-industrial aesthetic

Two-column layout with sidebar (TOC, prev/next nav, metadata panels).
Chrome bar header with rivets and type badges. Chamfered clip-path
containers for code blocks, blockquotes, images, and headings.
Gallery with viewfinder crosshairs for shots. Related fragments
section at bottom. Heading plates use asymmetric chamfer with
double-frame inner border matching the link tile design language.