Add birthday celebrations for students

Parents can set a birthday (MM-DD) per student in the Kids panel. During the student's birthday week (Mon–Sun), a festive overlay appears on login with balloons, birthday cake, confetti, and a personalized message — shown once per day via sessionStorage. Summer birthdays (June–Aug) show a note in admin suggesting the parent set an alternate school-week date to celebrate early.

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

public/index.html

@@ -110,6 +110,17 @@
     </div>
   </div>
 
+  <!-- Birthday celebration overlay -->
+  <div id="birthday-overlay" hidden>
+    <div id="birthday-content">
+      <div class="birthday-balloons" aria-hidden="true">🎈🎈🎈</div>
+      <div class="birthday-cake">🎂</div>
+      <h2 id="birthday-heading">Happy Birthday!</h2>
+      <p id="birthday-message" class="birthday-message"></p>
+      <button id="birthday-close" class="check-btn">Thanks! 🎉</button>
+    </div>
+  </div>
+
   <!-- Course detail: units + items -->
   <section id="view-course" class="view" hidden>
     <button class="back-btn" data-back="course">← Back</button>