Remove Angular, React becomes the app (migration 8/8)

[charityquinn-cognition]

Summary

Top of the stack, on #695. Deletes Angular and promotes the React build to the only build. Mostly deletions: 78 files, ~-11.9k lines (most of it yarn.lock).

• Removed src/ entirely (app/, environments/, index.html, main.ts, polyfills.ts, styles.scss, test.ts) plus angular.json, tslint.json, karma.conf.js, ngsw-config.json, browserslist, tsconfig.app.json, tsconfig.spec.json, and e2e/ (protractor).
• tsconfig.react.json → tsconfig.json, replacing the Angular one.
• Dependencies: dropped all @angular/*, the CLI/devkit/compiler-cli/language-service, rxjs + rxjs-compat, zone.js, tslib, node-fetch, unfetch, codelyzer, tslint, karma/jasmine/protractor and ts-node. Removed via yarn, so yarn.lock shrinks accordingly. Scripts lose their react: prefix and become the primary ones (dev/start/build/preview); test, lint and e2e are removed rather than left as commands that fail — there is no test tooling left in the repo (the Angular suite was the CLI default specs, which went with src/).
• Vite build.outDir moves dist/react → dist, which is what firebase.json hosting already serves; its ** → /index.html SPA rewrite needed no change.
• .travis.yml: Node 6.9 → 20, drops the global Angular CLI install, and runs yarn install --frozen-lockfile && yarn build. This also fixes the ERR_OSSL_EVP_UNSUPPORTED failure noted in React tooling scaffold (migration 1/8) #689 — the legacy Webpack 4 build that hit it no longer exists.
• README/CONTRIBUTING retargeted to React + Vite + Yarn, with the stale npm run precache / static-serve steps (scripts that don't exist) replaced by yarn build / yarn preview.

Not renamed: React source stays in src-react/ rather than moving to src/. A directory rename here would bury the actual removals in rename noise; it's a trivial follow-up if you'd prefer the conventional layout.

Remaining "angular" references are intentional: the package name, .firebaserc project, the deployed Firebase URL, the manifest's app name and the angular2-hn-assets cache name — all identity/branding, not tooling.

Verified: clean rm -rf node_modules && yarn install --frozen-lockfile resolves no Angular packages; yarn build emits dist/ with sw.js + manifest.json; yarn dev and yarn preview both serve; feed → comments → back → theme switch all still work after the removal.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/1ff25c6cf2f949458f82cc596fc79c65
Requested by: @charityquinn-cognition

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