style(toast): warning toast uses the /90 opacity variant

Matches the error/success/info entries visually. bg-yellow-500/90 is
generated by the Tailwind scanner from this file itself (content globs
include web/src/**/*.ts), same as the sibling bg-*-500/90 classes — no
safelist entry needed.

web/src/toast.ts

@@ -40,9 +40,7 @@ const getToastConfig = (type: ToastType) => {
       icon: "ℹ️",
     },
     warning: {
-      // bg-yellow-500 (no opacity modifier): it must already appear in the
-      // templates for the Tailwind build to emit the class.
-      bgClass: "bg-yellow-500",
+      bgClass: "bg-yellow-500/90",
       icon: "⚠️",
     },
   };