RHS table of contents is misaligned and hard to navigate

The right-hand-side table of contents is visually misaligned and makes it hard to jump between sections. Reported during the theme evaluation in QuantEcon/lecture-python-programming#475 (closed, never merged).

Context. Reproduced against main at 6a22984, which already contains the RHS ToC work (21cf054 Fix RHS ToC: show all heading levels, lower breakpoint to 900px, b41f31c Fix theme toggle, ToC scroll-spy, and breadcrumb dropdowns, and the scroll-spy parts of ee731ff). The defect survives those changes.

Preview build: https://699d241d064feca2f249791b--epic-agnesi-957267.netlify.app/intro

Reported symptoms: the navigation is "a bit off" positionally, and jumping to different sections is harder than it should be.

Suggested starting points: _toc.scss for the alignment, toc.js for scroll-spy and click-to-jump behaviour, and the interaction with the topbar after 6d6a1b4 (fix(topbar): remove max-width, align controls over RHS ToC) — the topbar and the ToC now share a horizontal band, so a regression in one can present as misalignment in the other. Worth pinning down whether "hard to jump" means small hit targets, incorrect anchor offsets under a sticky header, or scroll-spy fighting the user's scroll.

Start with _toc.scss and toc.js, then compare their behavior with the topbar changes from 6d6a1b4 and the reproduced preview at /intro. Determine whether the navigation problem is alignment, hit targets, anchor offsets, or scroll-spy behavior, and verify that the RHS ToC is visually aligned and reliably jumps between sections.