Move vitest onto vite 8 so @vitejs/plugin-react can update

[royalpinto007]

Why

@vitejs/plugin-react is stuck at ^4.3.1. Version 6 declares a peer of
vite ^8.0.0, and this project brings vite 5 transitively through
vitest ^1.6.0, so npm ci fails on the conflict:

npm error Conflicting peer dependency: vite@8.2.1
npm error   peer vite@"^8.0.0" from @vitejs/plugin-react@6.0.5

Dependabot re-proposes it and it fails the same way each time.

Scope

Move vitest to a release built on vite 8, then take @vitejs/plugin-react to 6.

• vitest ^1.6.0 to current
• @vitejs/plugin-react ^4.3.1 to ^6
• check vitest.config.ts against the newer config surface

Vitest 2 and 3 changed a few defaults that matter here: the environment
resolution, how globals are declared, and coverage provider names. The test
files themselves are likely fine, since they use plain describe/it/expect.

Done when

• npm ci succeeds without --legacy-peer-deps
• npx vitest run passes with the same test count as today
• tsc, eslint, prettier and next build still pass
• The dependabot ignore for @vitejs/plugin-react (sibling issue) is removed

Good first issue?

Not quite: it is small in diff but wants someone comfortable reading a vitest
migration guide and checking the config surface rather than only bumping numbers.