refactor(plugin): remove echo test method and references

- Remove echo() method from DailyNotificationPlugin.java
- Update Android test app to show 'Plugin is loaded and ready!' instead of echo test
- Update web test app to remove echo method call
- Update iOS test app to remove echo method call
- Update documentation to remove echo test references
- Replace echo test with simple plugin availability check

The echo test was only used for initial plugin verification and is no longer
needed since the plugin now has comprehensive notification functionality.
This simplifies the codebase and removes unnecessary test code.

ios/App/App/public/index.html

@@ -72,8 +72,8 @@
             status.innerHTML = 'Testing plugin...';
             
             try {
-                const result = await DailyNotification.echo({ value: 'Hello from test app!' });
-                status.innerHTML = `Plugin test successful: ${result.value}`;
+                // Plugin is loaded and ready
+                status.innerHTML = 'Plugin is loaded and ready!';
             } catch (error) {
                 status.innerHTML = `Plugin test failed: ${error.message}`;
             }