fix: use Tailwind v4's adaptive placeholder color in preflight

The preflight pinned input/textarea placeholders to the v3 gray-400 hex
(#9ca3af), which reads wrong on dark backgrounds and custom palettes.
Tailwind v4 derives it from the text color instead:
color-mix(in oklab, currentColor 50%, transparent).

Author: chrisbbreuer

packages/crosswind/src/preflight.ts

@@ -351,7 +351,7 @@ textarea {
 input::placeholder,
 textarea::placeholder {
   opacity: 1;
-  color: #9ca3af;
+  color: color-mix(in oklab, currentColor 50%, transparent);
 }
 
 button,