fix: add missing os_listener toggle to notification CSS editor

The os_listener field existed in the backend model but was never
exposed in the UI. It defaulted to false, so OS notifications were
captured to history but never fired the visual effect. Now shows
a toggle at the top of the notification section, defaults to ON
for new sources.

server/src/wled_controller/templates/modals/css-editor.html

@@ -375,6 +375,18 @@
 
                 <!-- Notification-specific fields -->
                 <div id="css-editor-notification-section" style="display:none">
+                    <div class="form-group">
+                        <div class="label-row">
+                            <label for="css-editor-notification-os-listener" data-i18n="color_strip.notification.os_listener">Listen to OS Notifications:</label>
+                            <button type="button" class="hint-toggle" onclick="toggleHint(this)" title="?" data-i18n-aria-label="aria.hint">?</button>
+                        </div>
+                        <small class="input-hint" style="display:none" data-i18n="color_strip.notification.os_listener.hint">When enabled, this source automatically fires when a desktop notification appears (Windows toast / Linux D-Bus). Requires the app to have notification access permission.</small>
+                        <label class="switch">
+                            <input type="checkbox" id="css-editor-notification-os-listener">
+                            <span class="slider"></span>
+                        </label>
+                    </div>
+
                     <div class="form-group">
                         <div class="label-row">
                             <label for="css-editor-notification-effect" data-i18n="color_strip.notification.effect">Effect:</label>