fix(ui): sidebar active fill overshoot, text shift, sticky TOC and token rhythm

[vshengbro]

Summary

Fixes five defects in the docs shell, all verified against a local build of the docs site.

Sidebar

• side-indent was 17px (margin 8 + padding 8 + border 1), but an active row keeps its own padding, so the fill landed 8px past the dashed guide. Corrected to 9px (margin + border) in both themes.
• That negative margin also dragged the label 9px left. It's now paid back with padding-left: calc(space-md + side-indent-num) — the two cancel for the text while the background still reaches the guide. Measured 0px text shift at root, depth-1 and depth-2, with row height held at 48px in all states.
• Hover already used inset box-shadow and never moved text; left as is.

TOC

• c_euv_doc_layout inherited flex-shrink: 1 as a flex item of the scroll container's column. It was therefore clamped to its min-height: 100% floor (824px) despite 12780px of content, and a sticky descendant can only travel inside its containing block — so the TOC unpinned after roughly one viewport of scroll. flex-shrink: 0 restores the full range; verified pinned at a constant 100px offset through 11000px of scroll.
• Pinned at padding-main-top rather than a bare 0, so it keeps the same breathing room below the container edge as the article.
• Active entries bold at 700; hover no longer bolds. The docs override set font-weight: 700 on :hover in both light and dark — that contradicted the rule and is corrected.

Markdown rhythm

• Replaced em-based heading/block margins with space-sm / gap-component, so article copy sits on the same token scale as the rest of the UI. Article spacing measured 16px against the example's 8px; now a uniform 8px.

Links

• .md-body a restyled itself on hover (dashed → solid underline, accent colour). Now one static treatment for both states.

Padding

• Deleted the padding-main-bottom token and both usages in c_app_main rather than zeroing it.

Verification

• cargo fmt --all -- --check clean
• cargo clippy --workspace --all-targets — 0 warnings
• cargo test --workspace --all-features — all suites passing
• Browser-measured on a locally built docs bundle at 1440×900: sidebar text x, fill overshoot, row height, TOC offset across scroll positions, and article rhythm

Version bumped to 0.28.20 with workspace path-dep pins realigned, since the sync workflows skip on PRs.

Notes

The docs deploy workflow is failing independently of this change — Install euv-cli fails to compile euv-cli v0.28.15 with cannot find value NO_CACHE_NO_STORE_MUST_REVALIDATE. That blocks any docs deploy, before and after this PR, and is not addressed here.