Modernize stack: Vite, React 19, RTK Saga, Tailwind, dual GH Pages + Workers deploy

[hyldmo]

Full rebuild of the 2023-era starter (React 16, Redux 4 + Saga, Router 5, Webpack, Less, Jest, Cypress, ESLint) on the current house stack.

Keep: Redux + Saga pattern, Tracker/Home/About demo, makeActionCreator util.
Replace:

• Webpack -> Vite 7, React 16 -> 19, Router 5 + connected-react-router -> Router 7 BrowserRouter (basename follows deploy base)
• Redux 4 -> Redux Toolkit 2 + react-redux 9 + redux-saga via typed-redux-saga (with the noRestrictedImports rule enforcing it)
• Less -> Tailwind 4, ESLint/Stylelint -> Biome 2.5 (shared rules + component-decl/fc-props/no-casts plugins), Jest -> Vitest + Testing Library, joi-browser -> Zod, classnames -> cn()
• Cypress 5 dropped in v1 (Playwright follow-up)

Deploy: one dist/ serves both targets — wrangler.jsonc Static Assets + SPA fallback and a thin /api stub Worker for Cloudflare, VITE_BASE + 404.html + deploy-pages workflow for GH Pages. CI checks lint + typecheck + test + both builds.

Verified: yarn check clean, yarn build and yarn build:pages emit correct asset paths, wrangler deploy --dry-run reads the assets.