Add emoji toggle setting to parent dashboard Settings panel

Parent can turn the floating school emoji on/off from Settings > Student home screen. Setting persists in the DB and is read via a public /api/public-settings endpoint on the student home page.

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

public/admin.html

@@ -535,6 +535,15 @@
             </label>
           </div>
 
+          <div class="field-group" style="margin-top:1rem">
+            <label>Student home screen</label>
+            <label class="toggle-label" style="margin-top:.5rem">
+              <input type="checkbox" id="setting-show-emoji" role="switch">
+              <span class="toggle-track"><span class="toggle-thumb"></span></span>
+              Show floating emoji on the kid picker screen
+            </label>
+          </div>
+
           <button type="submit" style="margin-top:1.25rem">Save settings</button>
         </form>