Phase 0: scaffold React (Vite + TS) app alongside Angular

[vibhaseshadri-cognition]

Summary

PR 1 of the Angular -> React migration. Adds a self-contained react/ app next to the Angular source (nothing under src/ is touched) so both coexist until parity is verified. No feature code yet — only tooling, global styles and a placeholder App that renders the theme wrapper (<div class="default"><div class="body-cover"/><div class="wrapper">…).

Stack: Vite 7 + React 19 + TypeScript (strict), React Router (react-router-dom v7, v6-compatible API), Vitest + React Testing Library (jsdom), Playwright (replaces Protractor), ESLint 9 flat config + Prettier (same singleQuote / tabWidth 4 / printWidth 120 / trailingComma es5 as root package.json), vite-plugin-pwa (Workbox generateSW, autoUpdate, manifest ported from src/manifest.json).

Notes on the SCSS port: _themes.scss now uses @use instead of @import, math.div($skull-size, 8) instead of /, and color.adjust(..., $lightness: -33%) instead of darken() — dart-sass removed/deprecated those forms; output is identical. The app-root:empty + .app-loader splash selector becomes #root:empty + .app-loader.

Also adds .github/workflows/react.yml (lint, typecheck, unit tests w/ coverage, build, Playwright e2e) scoped to react/** since the repo has no CI for the React side.

Angular -> React file mapping

Angular	React
src/styles.scss	react/src/styles/styles.scss
src/app/shared/scss/_media.scss, _theme_variables.scss, _themes.scss	react/src/styles/ (same names)
src/index.html	react/index.html
src/manifest.json	react/vite.config.ts (VitePWA.manifest)
ngsw-config.json	react/vite.config.ts (VitePWA.workbox)
src/assets/**, src/favicon.ico	react/public/assets/**, react/public/favicon.ico
e2e/ (Protractor)	react/e2e/ + react/playwright.config.ts
karma.conf.js / src/test.ts	react/vite.config.ts (test) + react/src/test/setup.ts
tslint.json	react/eslint.config.js

Verification

In react/: npm run lint, npm run typecheck, npm test (1 smoke test), npm run build (emits sw.js, manifest.webmanifest, 43 precached entries), npm run e2e (1 Playwright test) all pass.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/1e6f5b46badc4852993dc2299512cd3d
Open in Devin Desktop: https://app.devin.ai/desktop/session/1e6f5b46badc4852993dc2299512cd3d?variant=devin
Requested by: @vibhaseshadri-cognition

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