fix(web): remove mock echo function from web test app

- Remove echo function from mock DailyNotification plugin
- Align web version with Android version (no echo test)
- Ensure consistent behavior across all test platforms

This completes the echo test removal across all platforms:
- Android: Removed echo method from plugin and test app
- Web: Removed mock echo function
- iOS: Already updated in previous commit

All test apps now use direct notification testing without echo dependency.

www/index.html

@@ -70,9 +70,6 @@
         if (!window.DailyNotification) {
             console.log('Creating mock DailyNotification plugin...');
             window.DailyNotification = {
-                echo: function(options) {
-                    return Promise.resolve({ value: `Mock echo: ${options.value}` });
-                },
                 configure: function(config) {
                     console.log('Mock configure called with:', config);
                     return Promise.resolve();