fix(build): unbreak next build type check on vitest NODE_ENV

The deployment failed at "Running TypeScript ..." with:

  vitest.config.ts(7,13): error TS2540: Cannot assign to 'NODE_ENV'
  because it is a read-only property.

Next's next/types/global declares NODE_ENV on ProcessEnv as `readonly`.
tsconfig includes **/*.ts, so next build type-checks vitest.config.ts and
the direct assignment from 34017d8 took every deploy down with it.

Object.assign has no source-to-target assignability constraint, so it
compiles while still writing NODE_ENV through process.env's setter —
React still gets its development build, which is what exposes `act`.

Verified: tsc --noEmit clean, 3839 tests pass, npm run build succeeds.

vitest.config.ts

@@ -4,7 +4,14 @@ import path from "node:path";
 // React only exposes `act` in its development build, so a shell with
 // NODE_ENV=production would break every component test. Vitest only sets
 // NODE_ENV when it is unset, so force it.
-process.env.NODE_ENV = "test";
+//
+// Next's `next/types/global` declares NODE_ENV on ProcessEnv as `readonly`,
+// so a plain `process.env.NODE_ENV = "test"` fails the type check that
+// `next build` runs over this file (tsconfig includes **/*.ts) and takes
+// the whole deployment down. Object.assign has no source-to-target
+// assignability constraint, so it compiles while still writing the value
+// through process.env's setter at runtime.
+Object.assign(process.env, { NODE_ENV: "test" });
 
 export default defineConfig({
   resolve: {