ui: center login page, add logo and title

Login now fills the viewport (min-h-screen flex items-center/justify-center)
with the logo, "Artifacto" title, and tagline above the password form.
Drops the header/footer chrome on this page since there's nothing to link
to before sign-in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/templates/login.html

@@ -1,12 +1,18 @@
 {{ define "login" }}
 {{ template "head" "Log in" }}
-{{ template "header" false }}
-<main class="max-w-6xl mx-auto px-6 py-8">
-  <div class="max-w-sm mx-auto mt-16">
-    <h1 class="text-2xl font-semibold mb-4">Log in</h1>
+<main class="min-h-screen flex items-center justify-center px-6 py-12">
+  <div class="w-full max-w-sm">
+    <div class="flex flex-col items-center text-center mb-8">
+      <img src="/static/logo.png" alt=""
+           class="w-20 h-20 rounded-full ring-1 ring-slate-700 mb-4 shadow-lg shadow-indigo-500/10">
+      <h1 class="text-3xl font-semibold tracking-tight">Artifacto</h1>
+      <p class="text-sm text-slate-400 mt-1">self-hosted HTML artifact publisher</p>
+    </div>
+
     {{ if .Error }}
       <div class="rounded bg-red-900/40 border border-red-800 text-red-200 px-3 py-2 mb-3 text-sm">{{ .Error }}</div>
     {{ end }}
+
     <form method="post" action="/login" class="space-y-3">
       <input type="hidden" name="next" value="{{ .Next }}">
       <label class="block">
@@ -18,5 +24,6 @@
     </form>
   </div>
 </main>
-{{ template "footer" }}
+</body>
+</html>
 {{ end }}