fix(toc): keep active outline entry visible

What changed

• keep the active scrollspy entry inside the visible desktop outline
• mirror active state into the responsive inline outline
• recompute its position when the responsive outline opens
• add a focused regression test

Root cause

The scrollspy selected the correct heading, but the independently scrolling fixed outline stayed at scrollTop = 0. On long pages the active link and marker moved below the outline viewport, making the scrollspy appear inactive.

Validation

• bun run lint
• bun run build
• bun run typecheck
• bun test (542 passing)