fix(samples): make the vite config ESM so it loads on the sandbox Node

[gaelsimon]

With Vite pinned back to 7, samples still died in CodeSandbox: vite.config.js is ESM but the sample has no "type": "module", so Node treats it as CommonJS and tries to require() Vite's ESM entry. That only works from Node 20.19, and the sandbox runs 20.12.1 — hence ERR_REQUIRE_ESM.

Generating vite.config.mjs instead makes Node load it as ESM natively, on any version. The oxc block goes with it: it was added for Vite 8 and is inert on 7 — the React samples still transform through esbuild, verified in the rebuilt bundles.

Test: vite build and vite dev both pass under Node 20.12.1; the sandbox now only prints the version warning.

[claude]

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