fix: add type declarations for css export (TypeScript 6 compat)

[andershermansen]

What?

Add TypeScript type declarations for the ./css package export in @payloadcms/next.

Why?

TypeScript 6 enables noUncheckedSideEffectImports by default. This causes import '@payloadcms/next/css' — present in all three files generated by create-payload-app — to fail with:

error TS2882: Cannot find module or type declarations for side-effect import of '@payloadcms/next/css'.

The published ./css export currently has no types field:

"./css": {
  "import": "./dist/prod/styles.css",
  "default": "./dist/prod/styles.css"
}

How?

Add src/exports/css.ts as a regular TypeScript source file (consistent with all other exports so the existing build pipeline produces dist/exports/css.d.ts. Then wire the types field into both dev and publishConfig exports.

Testing:
Tested locally by copying the new files
cp ../../payload/packages/next/dist/exports/css.* node_modules/@payloadcms/next/dist/exports
and pacthing the package.json manually:

--- original-package.json       2026-04-22 10:43:26
+++ package.json        2026-04-22 10:43:57
@@ -24,6 +24,7 @@
   "exports": {
     "./css": {
       "import": "./dist/prod/styles.css",
+      "types": "./dist/exports/css.d.ts",
       "default": "./dist/prod/styles.css"
     },
     "./withPayload": {

after that my test project builds fine with typescript 6.0.3

Fixes #16346

[andershermansen]

After the build of the reproduction repo with manual patching the production run pnpm start also works fine in my testing. That is it shows the correct layout.