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/index.html

@@ -8,6 +8,18 @@
 </head>
 <body>
 
+  <!-- Background decorations -->
+  <div class="page-confetti" aria-hidden="true">
+    <span style="left:4%;top:12%;animation-duration:4.2s;animation-delay:0s">⭐</span>
+    <span style="left:89%;top:7%;animation-duration:5.1s;animation-delay:1.3s">📚</span>
+    <span style="left:13%;top:78%;animation-duration:3.8s;animation-delay:2.2s">✏️</span>
+    <span style="left:80%;top:72%;animation-duration:4.7s;animation-delay:0.6s">🌈</span>
+    <span style="left:47%;top:4%;animation-duration:5.5s;animation-delay:1.9s">🎨</span>
+    <span style="left:93%;top:44%;animation-duration:3.5s;animation-delay:3.1s">⭐</span>
+    <span style="left:2%;top:52%;animation-duration:4.9s;animation-delay:0.3s">🌟</span>
+    <span style="left:60%;top:90%;animation-duration:4.4s;animation-delay:2.7s">📝</span>
+  </div>
+
   <!-- Kid picker -->
   <section id="view-home" class="view">
     <h1 class="app-title">🎓 Homeschool</h1>