Phase 1: React 18 + Vite + TypeScript scaffold and shared infrastructure

[vibhaseshadri-cognition]

Summary

First phase of the Angular → React migration. Adds a self-contained React app under web/ (Angular in src/ is untouched and still builds) plus a GitHub Actions workflow running lint, test, build for web/. Later phases port the feature components on top of this shared infrastructure and only then delete the Angular source.

What landed:

• Stack: React 18 + TS, Vite 5, Vitest + React Testing Library (jsdom), ESLint 9 flat config + Prettier reusing the Angular repo's prettier settings (tabWidth: 4, singleQuote, printWidth: 120, trailingComma: es5).

• Models (src/models/) ported from src/app/shared/models/ as interfaces; fields the API omits (url, domain, poll, about, …) are optional so strict mode is usable.

• api/hackerNews.ts replaces HackerNewsAPIService — Observables become promises over native fetch, base URL unchanged. Poll aggregation is preserved but now awaits all options instead of firing detached subscriptions:

  const story = await lazyFetch<Story>(`${BASE_URL}/item/${id}`);
  if (story.type === 'poll' && story.poll) {
      const options = await Promise.all(story.poll.map((_, i) => fetchPollContent(story.id + i + 1)));
      story.poll = options;
      story.poll_votes_count = options.reduce((votes, o) => votes + o.points, 0);
  }

  lazyFetch additionally rejects on non-2xx so components can render the error state instead of parsing an error body.

• context/SettingsContext.tsx replaces the SettingsService singleton: same localStorage keys (theme, titleFontSize, listSpacing, openLinkInNewTab) and the same prefers-color-scheme behaviour — subscribe to the media query, and on start-up fall back to night/default from the query only when no theme was saved.

• utils/formatCommentCount.ts replaces the comment pipe; components/Loader / ErrorMessage port the shared components with their SCSS.

• Styles: _media, _theme_variables, _themes copied over ($x / n → math.div so the build is warning-free); the three themes apply through the App wrapper div driven by settings.theme.

27 tests pass; npm run lint and npm run build are clean.

Note: this PR targets COG-GTM/angular2-hn because pushing to ankehao-demo/angular2-hn returns 403 (no write access); both repos are at the same commit.

Link to Devin session: https://app.devin.ai/sessions/6382e57ba3ff4a388ee6e9bd0ad2d37d
Requested by: @vibhaseshadri-cognition

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