chore: Replace npm deps with native browser APIs

[reakaleek]

What

Replace npm dependencies in src/Elastic.Documentation.Site with native browser APIs or small local utilities, plus one JS behavior CSS already covers. Each dep is used for one narrow purpose the platform already provides.

Changes

ua-parser-js → navigator.userAgentData / regex

• Used only to detect macOS (⌘ vs Ctrl hint) and Chrome (one <details> quirk).
• Replace with navigator.userAgentData?.platform + /mac/i.test(navigator.userAgent) / /chrome/i.test(navigator.userAgent).
• Files: Assets/main.ts:53,187, Assets/open-details-with-anchor.ts, Assets/web-components/NavigationSearch/SearchInput.tsx

@uidotdev/usehooks → local useDebounce

• Entire hooks library imported for a single useDebounce hook (3 call sites).
• Add an ~8-line local hook (useState + useEffect + setTimeout/clearTimeout).
• Files: Assets/web-components/{ModalSearch,NavigationSearch,FullPageSearch}/use*Query.ts

lodash + @types/lodash → local throttle

• Full lodash imported for one throttle call in Assets/pages-nav.ts:1,91.
• Add a ~6-line local throttle function or use import throttle from 'lodash/throttle' as an intermediate step before dropping the dep entirely.

smooth-scroll.ts → CSS scroll-behavior

• The in-page anchor smooth-scroll handler reimplements what CSS ships: html { scroll-behavior: smooth } + scroll-margin-top on targets (no scroll-behavior is currently defined). The browser updates the hash natively on anchor clicks.
• Delete Assets/smooth-scroll.ts and its registration. (~17 lines)

select-dom → local $/$$ helpers (optional)

• Thin wrapper over querySelector/querySelectorAll used across 8 files.
• A 4-line local helper removes the dep: const $ = (sel, root = document) => root.querySelector(sel).
• 21 call sites — more churn than the others; do last or skip if the savings don't justify it.

Part of the broader ponytail over-engineering audit tracked in #3574.

[reakaleek]

Part of the ponytail over-engineering audit tracked in #3574.