feat: React migration - Step 1 & 2: Project setup + routing

[devin-ai-integration]

Summary

Scaffolds a new React application inside react-app/ as the first phase of migrating the Angular 2 Hacker News PWA to React. This PR covers Step 1 (project setup) and Step 2 (routing) only — page components are placeholders that will be implemented in follow-up PRs.

What's included:

• Vite + React 19 + TypeScript project with sass and react-router-dom v6
• TypeScript interfaces migrated from Angular classes (Story, Comment, User, PollResult, Settings, FeedType)
• React Router v6 config mirroring the Angular route structure (/news/:page, /newest/:page, /show/:page, /ask/:page, /jobs/:page, /item/:id, /user/:id)
• React.lazy() for ItemDetails and UserProfile routes
• SCSS theme system (_themes.scss, _theme_variables.scss, _media.scss) ported from Angular
• App shell with theme class wrapper, Header nav, Footer, and <Outlet />

Local Testing Screenshots

Verified locally — routing, redirects, and lazy loading all work as expected:

News feed route (/news/1) — root / correctly redirects here:

Item details route (/item/123) — lazy loaded:

User profile route (/user/test) — lazy loaded:

Review & Testing Checklist for Human

• Route structure parity: Verify the routes in react-app/src/router.tsx match src/app/app.routes.ts — especially the redirect from / → /news/1, feed pagination routes, and lazy-loaded item/user routes
• Header divergence: The React Header.tsx uses a simplified <NavLink> nav (visible in screenshots — links run together without spacing). The Angular header includes a logo SVG, settings cog icon, and different markup structure. Confirm whether this simplified version is acceptable for now or needs closer parity
• Type accuracy: Story.time_ago is typed as string in React but number in the Angular story.ts. The API likely returns a string — confirm this is intentional. Also note the crated_time typo in User is preserved from the original
• Build locally: Run cd react-app && npm install && npm run build to confirm clean compilation. SCSS deprecation warnings are expected (inherited from Angular theme system)

Suggested test plan: Start the dev server (cd react-app && npm run dev), verify / redirects to /news/1, click through each nav link (news → newest → show → ask → jobs), navigate to /item/123 and /user/test directly, and confirm lazy loading works (check network tab for chunk loading).

Notes

• Vite template boilerplate (README.md, vite.svg, react.svg) is still present — can be cleaned up in a follow-up
• index.html title is "react-app" — should be updated to match the app name
• SCSS uses deprecated @import syntax and / division — builds with warnings but will need modernization before Sass 3.0
• No CI checks are configured for this repo

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Link to Devin run: https://app.devin.ai/sessions/2dcac70defc9410a99cfccf64cf16e3e
Requested by: @eashansinha

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