- Convert .apropos-toc <nav> to <details class="toc" open> in all three templates
- Add caret-down icon to summary (visible only on mobile via media queries)
- Desktop (≥768px): sticky sidebar via CSS grid, force-open via pointer-events:none, hide caret
- Mobile (≤767px): single column, collapsible TOC with rotating caret, margin-top below search bar
- Rename .apropos-toc-link to .toc-sidebar-link
- Merge 900px and 600px breakpoints into single 600px mobile media query
- Rename apropos.css → content-page.css; update 3 controller references