Liven up parent and student login screens

Parent: floating card with box-shadow over a soft multi-radial gradient background; decorative emoji header above PIN form.
Student: each kid card gets a colored top border matching their saved theme; subtle floating emoji (stars, books, pencil, rainbow) drift in the background.

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

public/admin.html

@@ -9,6 +9,7 @@
 <body>
 
   <section id="pin-gate" class="view parent">
+    <div class="pin-deco" aria-hidden="true">📚 ✏️ 🎓</div>
     <h1>👩‍🏫 Parent Dashboard</h1>
     <form id="pin-form">
       <input id="pin-input" type="password" inputmode="numeric" placeholder="Enter PIN" aria-label="Parent PIN">