Migrate React oidc-client-ts example to Vite, React 19 and react-oidc-context 3

[Wictorgirardi]

Depends on #28: this branch includes its commit, so review the last commit only until #28 is merged.

Summary

• Toolchain: Create React App → Vite 8, React 19.3, TypeScript 6.0, Tailwind CSS 4 (configured in CSS), ESLint 10 flat config and Prettier.
• Auth libraries: react-oidc-context 2 → 3.3 and oidc-client-ts 2 → 3.5.
• Config:
  • Comes from .env (VITE_OIDC_ISSUER_URI, VITE_OIDC_CLIENT_ID), which holds the public hosted demo values.
  • .env.local.sample targets the local Keycloak from Add shared CI workflows, local Keycloak and contributing guide #28.
• Auth fixes:
  • Redirects back to the current origin instead of the hardcoded production URL.
  • Logs out with signoutRedirect, so the Keycloak session ends too. It used to call removeUser, which only cleared local state.
  • Auth errors are now shown; before, that branch was unreachable.
• UI:
  • Shows decoded access and ID tokens and drops the refresh-token panel.
  • Fixes the broken background fallback image and the GitHub link, which now points to this example.
  • Icons are inline SVG, so @iconify/react is no longer needed.
• Tooling:
  • pnpm 10.34 via packageManager, Node 24 via .nvmrc and engines.
  • vercel.json pins the Vite preset, since the Vercel project was set up for CRA.
• Workflow: now calls the shared CI and Vercel deploy workflows.

Test plan

• pnpm install --frozen-lockfile && pnpm typecheck && pnpm lint && pnpm build pass on Node 24.
• tools/e2e-smoke passes against the local Keycloak: log in, decoded tokens shown, log out.
• Vercel preview deploy. It's skipped for PRs from forks, so it first runs after merge.

[Wictorgirardi]

Combined into #44 with the other SPA examples. The changes are the same; only the commit SHAs differ.