refactor: replace harsh shadows with subtle ring+shadow, add heading-serif and border-hairline

templates/login.gohtml

@@ -2,7 +2,7 @@
 {{define "content"}}
 <div class="flex min-h-screen items-center justify-center p-4">
   <div class="w-full max-w-md">
-    <h1 class="mb-2 text-3xl font-normal text-foreground">Sign in</h1>
+    <h1 class="heading-serif mb-2 text-3xl font-normal text-foreground">Sign in</h1>
     <p class="text-foreground-muted mb-8">Enter your credentials to continue.</p>
 
     <form method="POST" action="/login" class="flex flex-col gap-6">
@@ -44,4 +44,4 @@
     {{end}}
   </div>
 </div>
-{{end}}
+{{end}}