refactor(input): R10 S3 review — fold Welcome's local inputCls into the shared recipe

Welcome kept the last local `const inputCls` (rounded-xl, py-2.5) after S3b consolidated
the other three, leaving its auth fields the sole inputs off the unified rounded-lg recipe.
Import the shared inputCls and drop the local const — now every input is truly one recipe.
(Auth fields go rounded-xl→rounded-lg, py-2.5→py-2 to match.)

frontend/src/components/Welcome.tsx

@@ -22,6 +22,7 @@ import { api } from "../lib/api";
 import { HttpError } from "../lib/api";
 import { setLanguage, type LangCode } from "../i18n";
 import LanguageSwitcher from "./LanguageSwitcher";
+import { inputCls } from "./ui/form";
 
 const EMAIL_RE = /^[^@\s]+@[^@\s]+\.[^@\s]+$/;
 const PASSWORD_MIN = 10;
@@ -430,8 +431,6 @@ function AuthCard() {
     }
   }
 
-  const inputCls =
-    "w-full bg-surface border border-border rounded-xl px-3 py-2.5 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40";
   const titleKey = TITLE_KEY[mode];
 
   return (