fix(#3726): highlight last TOC heading when scrolled to bottom

[twjeffery]

Summary

• Fixes the sticky TOC highlighting the wrong section for short sections near the bottom of the page (e.g. "Get started" on the workspace page)
• Adds a bottom-of-page check in handleScroll: when window.innerHeight + window.scrollY >= document.body.scrollHeight - 10, the last visible heading is set as active instead of relying on the 120px threshold

Screen.Recording.2026-03-31.at.3.07.21.PM.mov

Closes #3726

Test plan

• Navigate to a docs page with a short final section (e.g. workspace example page)
• Scroll to the very bottom of the page — the last TOC item should be highlighted
• Click the last TOC item — it should become active
• Scroll back up — TOC highlighting should still work normally for all other sections

https://claude.ai/code/session_01Rcc9hcmnjnE8vdXhiz7VMM

[Spark450]

I think it is certainly working better but it looks like the selection "bounces" a bit then settles.
https://jam.dev/c/08f2e673-e30e-4d7d-a609-a1a81e62637a

[twjeffery]

I think it is certainly working better but it looks like the selection "bounces" a bit then settles. jam.dev/c/08f2e673-e30e-4d7d-a609-a1a81e62637a

I think that's expected for what it's fixing and doing. This is a fix to make it change (override what's set from the scroll position) to the last item if scroll is at the bottom. It's kind of like, it follows the heading based on the scroll position, and then if the scroll is at the bottom, then set it to bottom as active.