fix: Sign In button href uses BASE_URL prefix for /chat/ deployment

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/pages/Home.tsx

@@ -222,7 +222,7 @@ function Nav() {
           ))}
           <div className="w-px h-5 mx-1" style={{ background: "#e7e0d5" }} />
           <a
-            href="/login"
+            href={`${import.meta.env.BASE_URL}login`}
             className="px-3 py-2 rounded-lg text-sm font-semibold transition-colors"
             style={{
               background: "#14532D",