SonarCloud modernization (3/3): gas-installer & deploy HTML #444

Split from #421 (×68 modernization epic) — story 3 of 3. Behavior-preserving SonarCloud idiom modernizations across the installer and deploy front-end (JS + HTML, incl. one CSS-contrast fix). No new tests; existing Jest suite + Playwright specs + npm run check are the regression guard.

Files & fixes

• gas-installer/src/index.js — endsWith, optional chaining, misc idioms
• gas-installer/Index.html — String#replaceAll, optional chaining
• deploy/index.html — globalThis, Number.parseInt, String#replaceAll, optional chaining, nested-ternary extraction, remove unused safeId, CSS contrast

Test command

npm test (Jest), npm run check (prettier + eslint). Playwright specs must remain green.

Parent: #421 · Epic: #440