feature: migrate Angular 2 HN to React + TypeScript (Vite)

[devin-ai-integration]

Summary

Parallel rewrite of the Hacker News PWA client from Angular to React 18 + TypeScript + Vite, living in react-app/. The Angular source (src/, angular.json, karma/tslint/tsconfig, e2e/, root package.json/yarn.lock, ngsw-config.json) is removed in the final commit, after the React build was verified in the browser.

Preserved contracts

• Routes (react-router-dom v6): / → /news/1; /{news,newest,show,ask,jobs}/:page (bare /news → /news/1); /item/:id and /user/:id are React.lazy + Suspense (mirrors the Angular lazy modules).
• HN API (src/services/hackerNewsApi.ts): same base URL https://node-hnapi.herokuapp.com, same shapes — fetchFeed(type, page), fetchItemContent(id) (incl. poll option fan-out on story.id + i, summing poll_votes_count), fetchUser(id). RxJS replaced with fetch + AbortSignal; pages abort in-flight requests on unmount.
• SettingsService → SettingsContext (localStorage keys theme, openLinkInNewTab, titleFontSize, listSpacing; prefers-color-scheme listener picks night by default). Themes default / night / amoledblack applied as class on the app root; SCSS copied with original class names.
• CommentPipe → utils/comment.ts; index.html keeps the app-loader SVG, skip link, meta/theme-color, GA snippet.
• PWA via vite-plugin-pwa (autoUpdate, manifest from src/manifest.json, precache js/css/html/ico/png/svg, SPA navigateFallback).

Tooling: strict tsconfig, ESLint + Prettier (format:check), Vitest + RTL (6 files / 9 tests: API service, comment util, Item, Settings, FeedPage, App routing), source maps, browserslist. .travis.yml runs lint/test/build in react-app/ on Node 20; firebase.json hosting public → react-app/dist.

Verification: npm run build, npm run lint, npm run format:check, npm test all pass; all feed routes, item page, user page and all three themes checked at http://localhost:5173.

Note: the upstream node-hnapi.herokuapp.com/user/:id endpoint currently returns 404 for every user, so /user/:id shows the (correctly rendered) error state for both the old and new app.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/a3a619343bf04f1f90d7b8550f5dff9e
Open in Devin Desktop: https://app.devin.ai/desktop/session/a3a619343bf04f1f90d7b8550f5dff9e?variant=devin
Requested by: @ishita-cog

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Note

Devin errored when opening this Pull Request as ishita-cog.
As a fallback, Devin opened this PR as itself.

[devin-ai-integration]

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