chore(deps): upgrade vite to v8 and @vitejs/plugin-react to v6

Vite 8 replaces Rollup with Rolldown as the bundler engine. This
requires migrating build.rollupOptions to build.rolldownOptions and
the removed object-form manualChunks to the new codeSplitting.groups
API.

Migrate vite.config.ts:
- rollupOptions -> rolldownOptions
- manualChunks object -> codeSplitting.groups with RegExp test patterns
  (vendor: react/react-dom/react-router, i18n: i18next packages)

@vitejs/plugin-react@6 requires vite@^8 as peer dependency, so both
upgrades must land together.

Verified: npm run build succeeds (chunks: vendor 230KB, i18n 55KB),
tsc --noEmit clean, 339/339 vitest tests passing.

Author: jinglemansweep