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.

docs/testing-quick-reference.md

@@ -124,7 +124,6 @@ adb logcat -c
 
 ### Test Execution
 - [ ] Plugin loads successfully
-- [ ] Echo method works
 - [ ] Notification scheduling works
 - [ ] Background operation works
 - [ ] Force stop behavior is correct
@@ -147,7 +146,7 @@ adb logcat -c
 ## 📊 Success Criteria
 
 ### ✅ Test Passes When:
-- Plugin loads and responds to echo
+- Plugin loads and is ready for use
 - Notifications appear at scheduled time
 - Background operation works correctly
 - Force stop behaves as expected