- Replaces Map.groupBy (ES2024) with manual reduce for browser compat
- Promotes displayedActiveId to createMemo
- Adds scrollend listener cleanup on unmount
- Adds aria-current='location' to active TOC items
- Adds aria-label to mobile toggle button
- Increases mobile touch target (btn-sm → btn, full width)
- Bumps mobile dropdown z-index from z-30 to z-40
- Removes redundant 'relative' class on mobile container
- Scopes mobile toggle test query with within()

When prefers-reduced-motion is active, scrollIntoView uses behavior
'instant' which completes synchronously. The scrollend event may not
fire in all browsers for instant scrolls, leaving scrollingTo stuck.
Uses requestAnimationFrame to clear in the next frame instead.

- Fixes closing tag indentation in two-column layout wrapper
- Adds animate-pulse to prefers-reduced-motion override
- Adds test verifying requestAnimationFrame is used for instant scroll

Object.defineProperty on window.innerHeight, scrollY, and
scrollHeight leaked between tests. Captures original values
and restores in afterEach to prevent cross-test contamination.

Native scrollIntoView({behavior: 'smooth'}) can be overridden by
browser settings (Vivaldi, some Chromium forks disable smooth scroll
at the engine level). Replaces with requestAnimationFrame-driven
eased scroll that works regardless of browser scroll settings.
Still uses scrollIntoView for instant scroll (reduced-motion).

The scroll listener that shrinks/expands the header was firing during
programmatic TOC scrolling, causing a feedback loop: scroll crosses
50px threshold → header resizes → content shifts → scroll position
changes → header resizes again. Uses a data-scrollLock attribute to
suppress header state updates during TOC-driven scroll animations.
Dispatches a synthetic scroll event on completion to sync final state.

Shrink at scrollY > 50, expand only at scrollY < 10. The 40px dead
zone absorbs the ~32px content shift from header padding change,
preventing the layout feedback loop where header resize triggers
scroll position change which re-triggers header resize.

Adds 'dependencies' to SETTINGS_SECTIONS (13 entries) to match
upstream's new Dependencies section in SettingsPage.