Tune the React bundles after the Vite 8 / Rolldown upgrade #1462

The Vite 8 upgrade in #1461 grew the initial load of both React apps. Admin went from 225.5 to 247.5 KiB gzip and the dashboard from 226.1 to 251.7 KiB. Rolldown splits the entry into 44 (admin) and 67 (dashboard) separately gzipped files, where Rollup produced one, and zod 4 adds about 26 kB to admin's lazy form chunks.

Two quick attempts didn't help: switching the minifier back to esbuild made the output larger, and grouping the icons didn't change the initial load. Worth a proper look:

• Use Rolldown's advancedChunks (output.advancedChunks.groups) to group the vendor graph (React, Radix, TanStack, i18next) instead of relying on the defaults.
• Check whether the eagerly imported en-US catalogs can be split per route namespace.
• Measure with npx vite-bundle-visualizer or rolldown's own report before and after.

Done when initial-load gzip for both apps is back at or below the pre-upgrade numbers, or we have a written reason why it can't be.