Phase 3: PWA service worker, Playwright e2e, 100% coverage and Angular removal

[vibhaseshadri-cognition]

Summary

Final phase of the Angular → React migration: the React app moves from web/ to the repo root, the Angular app and its
toolchain are deleted, and the PWA + e2e story is restored on the React stack.

This branch is based on Phase 1 (#570) and already contains the four Phase 2 ports (#571 #572 #573 #574) merged in, so
its diff shrinks to just the Phase 3 work once those land.

PWA. vite-plugin-pwa replaces the Angular service worker; the manifest is expressed inline in vite.config.ts
with the same fields src/manifest.json had (name, four android-chrome-* icons, #b92b27 theme colour, standalone /
portrait, ./?utm_source=web_app_manifest start url). Workbox precaches the app shell and adds runtime caching for the
API:

navigateFallback: 'index.html',
runtimeCaching: [{
    urlPattern: /^https:\/\/node-hnapi\.herokuapp\.com\/.*$/,
    handler: 'NetworkFirst',
    options: { cacheName: 'hn-api', networkTimeoutSeconds: 10, expiration: { maxEntries: 100, maxAgeSeconds: 86400 } },
}]

index.html regains the full Angular head (description, OG/twitter cards, theme-color, msapplication tiles, apple-touch
icons, mask-icon, the #skip link, .app-loader splash and the <noscript> fallback) plus the inline Google Analytics
snippet that App.tsx's pageview tracking calls into.

E2E. Protractor is replaced by Playwright (e2e/app.spec.ts) running against the production build via
vite preview. Every node-hnapi.herokuapp.com request is fulfilled from e2e/fixtures.ts, so the suite is
deterministic and works offline; it covers the / → /news/1 redirect, feed navigation from the header, pagination
(start=31, prev/More visibility), opening an item and collapsing a nested comment, opening a user from a comment, and
the settings modal incl. theme switching persisted across a reload.

Coverage. npm run test:coverage is now 100% statements/branches/functions/lines over src/. Four tests were added
for the last gaps: a feed response that resolves after unmount, UserPage on a route without an :id, a poll whose
poll_votes_count is missing, and switching the theme back to default.

Angular removal. Deleted src/app, e2e/ (Protractor), angular.json, karma.conf.js, ngsw-config.json,
tslint.json, browserslist, the Angular tsconfig*.json, yarn.lock, .travis.yml and the root package.json
(@angular/*, rxjs, zone.js, unfetch, Karma/Jasmine). web/* moved up to the root, so npm ci && npm run dev
works from a fresh clone and firebase.json's public: dist keeps pointing at the Vite output. CI now runs
lint/test/build plus a second e2e job; README and CONTRIBUTING describe the React stack.

Verified locally: npm run lint, npm test (104 passing), npm run build, npm run test:e2e (6 passing).

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

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