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/admin.html

@@ -54,6 +54,9 @@
                   <option>🐼</option><option>🦖</option><option>🐙</option><option>🦉</option>
                   <option>🐻</option><option>🐰</option><option>🦁</option><option>🐶</option>
                 </select>
+                <label style="font-size:.85rem;color:#666;margin-top:.25rem">🎂 Birthday (optional)
+                  <input id="new-student-birthday" type="date" aria-label="Birthday" style="margin-top:.25rem">
+                </label>
                 <button type="submit">Add kid</button>
               </form>
             </div>