Rebuild oidc-spa tutorial starter on Vite and React 19

[Wictorgirardi]

Depends on #28: this branch includes its commit, so review the last commit only until #28 is merged. It pairs with #30 (oidc-spa example).

Summary

The blog post Securing Keycloak with OIDC SPA and Phase Two links here as its starting point.

• What it is now: exactly the oidc-spa example from Migrate React oidc-spa example to Vite, React 19 and oidc-spa 10 #30 minus the authentication code. Same Vite 8 / React 19 / TypeScript 6 / Tailwind 4 scaffold, same layout and assets, with .env already pointing at the demo realm. It used to be a CRA copy.
• src/Auth.tsx: a clear placeholder. Before, it was a stub that wasn't even exported.
• README: replaced the text copied from the oidc-client-ts example with the tutorial steps for oidc-spa 10:
  • pnpm add oidc-spa zod, then src/oidc.ts;
  • the Vite plugin;
  • the OidcInitializationGate;
  • the Auth.tsx component.
• Workflow: new CI-only workflow. The starter has no deployment.

Test plan

• pnpm install --frozen-lockfile && pnpm typecheck && pnpm lint && pnpm build pass on Node 24.
• diff -r against Migrate React oidc-spa example to Vite, React 19 and oidc-spa 10 #30 shows only the auth files, vercel.json, dependencies and titles.
• Applied the README's tutorial steps to a copy of the starter: it typechecks, builds, and logs in and out against the local Keycloak.

[Wictorgirardi]

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