Checkout: forward only the checkout fields, show clear errors, fix the contact form (#16)

Co-authored-by: Michael Czechowski <mail@librete.ch>

tests/unit/support/register.mjs

@@ -0,0 +1,6 @@
+// Loaded by `npm test` through `node --import`. Node runs the .ts sources directly (type stripping), but unlike Vite and Nitro it does
+// not resolve extensionless relative imports such as `import { … } from "./cmsError"` in a server/utils module.
+// The hook below retries such an import from a .ts file with ".ts" appended. Test files import sources with explicit .ts paths.
+import { register } from "node:module";
+
+register("./resolve-ts.mjs", import.meta.url);