Admin lock: replace explanation paragraph with centered key input

internal/web/templates/admin.html

@@ -1,20 +1,17 @@
 {{template "head" .}}
 {{template "topbar" .}}
-<div class="page">
-  <div class="float">
-    <div class="settings-head">
+<div class="center">
+  <div class="float unlock-card">
+    <div class="inner">
       <h1>Admin</h1>
-    </div>
-    <div class="settings-body">
-      <p>Enter the admin key to manage server settings. The key is kept in
-      sessionStorage for this tab only and is sent as a request header — it is
-      never stored in a cookie, so it will not accompany normal paste requests.</p>
       <form id="admin-key-form">
-        <label for="admin-key">Admin key</label><br>
-        <input type="password" id="admin-key" autocomplete="off" style="width:100%">
-        <button type="submit">Unlock</button>
+        <div class="pw-field">
+          <input type="password" id="admin-key" placeholder="Admin key" autocomplete="off" autofocus>
+        </div>
+        <button class="btn" type="submit">Unlock</button>
         <span id="admin-key-status"></span>
       </form>
+
       <div id="admin-panel" style="display:none">
         <h2>Settings</h2>
         <form id="admin-settings-form">