refactor(android)!: restructure to standard Capacitor plugin layout

Restructure Android project from nested module layout to standard
Capacitor plugin structure following community conventions.

Structure Changes:
- Move plugin code from android/plugin/ to android/src/main/java/
- Move test app from android/app/ to test-apps/android-test-app/app/
- Remove nested android/plugin module structure
- Remove nested android/app test app structure

Build Infrastructure:
- Add Gradle wrapper files (gradlew, gradlew.bat, gradle/wrapper/)
- Transform android/build.gradle from root project to library module
- Update android/settings.gradle for standalone plugin builds
- Add android/gradle.properties with AndroidX configuration
- Add android/consumer-rules.pro for ProGuard rules

Configuration Updates:
- Add prepare script to package.json for automatic builds on npm install
- Update package.json version to 1.0.1
- Update android/build.gradle to properly resolve Capacitor dependencies
- Update test-apps/android-test-app/settings.gradle with correct paths
- Remove android/variables.gradle (hardcode values in build.gradle)

Documentation:
- Update BUILDING.md with new structure and build process
- Update INTEGRATION_GUIDE.md to reflect standard structure
- Update README.md to remove path fix warnings
- Add test-apps/BUILD_PROCESS.md documenting test app build flows

Test App Configuration:
- Fix android-test-app to correctly reference plugin and Capacitor
- Remove capacitor-cordova-android-plugins dependency (not needed)
- Update capacitor.settings.gradle path verification in fix script

BREAKING CHANGE: Plugin now uses standard Capacitor Android structure.
Consuming apps must update their capacitor.settings.gradle to reference
android/ instead of android/plugin/. This is automatically handled by
Capacitor CLI for apps using standard plugin installation.

test-apps/android-test-app/settings.gradle

@@ -0,0 +1,27 @@
+include ':app'
+// Note: capacitor-cordova-android-plugins is not needed for standalone Android test app
+// It's only generated by Capacitor CLI for Capacitor apps
+
+// Include Capacitor Android (required for plugin)
+// Try to find Capacitor from the Vue test app's node_modules
+def capacitorPath = new File(settingsDir, '../daily-notification-test/node_modules/@capacitor/android/capacitor')
+if (capacitorPath.exists()) {
+    include ':capacitor-android'
+    project(':capacitor-android').projectDir = capacitorPath
+} else {
+    throw new GradleException("Capacitor not found at ${capacitorPath.absolutePath}. Please run 'npm install' in test-apps/daily-notification-test/ first.")
+}
+
+// Reference plugin from parent directory (for local development)
+// Path: test-apps/android-test-app/../../android = root/android
+// settingsDir = test-apps/android-test-app/
+// settingsDir.parentFile = test-apps/
+// settingsDir.parentFile.parentFile = root project directory
+def pluginPath = new File(settingsDir.parentFile.parentFile, 'android')
+if (pluginPath.exists() && new File(pluginPath, 'build.gradle').exists()) {
+    include ':daily-notification-plugin'
+    project(':daily-notification-plugin').projectDir = pluginPath
+} else {
+    throw new GradleException("Plugin not found at ${pluginPath.absolutePath}. Please ensure the plugin is at the correct location.")
+}
+