Replace Next.js with Vite + client-side router (TanStack Router or React Router)

[aldbr]

Description

DiracX-Web is a fully client-side SPA shipped as a static export. We use none of Next.js's server features (no SSR, no API routes, no middleware, no server components). Every component is marked "use client".

Next.js is increasingly server-oriented — each upgrade risks new server defaults we must opt out of. We're fighting the framework rather than benefiting from it.

What Next.js costs us today:

• Webpack config hacks for HMR in the monorepo (next.config.js)
• "use client" on every component, dynamic: "error", images: { unoptimized: true } — all workarounds for features we don't use
• Slower dev server than esbuild-based alternatives
• Extension boilerplate: adding a single custom app (e.g. gubbins) requires ~8 framework files (layouts, pages, error boundaries) on top of the ~4 actual business logic files

Proposal: Replace Next.js with Vite as build tool. Use this as an opportunity to simplify the extension contract so extensions only provide a config + components, not duplicated framework boilerplate.

Definition of Done

• Replace Next.js with Vite
• Evaluate whether TanStack Router is needed (we only have 2 routes: /auth and /)
• Simplify the extension contract to remove framework boilerplate
• OIDC redirect flow still works
• All existing Cypress tests pass

Thanks @ryuwd for the initial suggestion and investigation in LHCb context!

[aldbr]

ddev:
TanStack routers might not be needed considering we have only 2 pages (/auth and /)
We agree on Vite