feat: 안드로이드 하이브리드 앱(Capacitor) 구성

- capacitor.config + android/ 네이티브 프로젝트
- 앱 전용 빌드(.env.capacitor, build:app), 뒤로가기·safe-area
- docs/ANDROID.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

android/app/src/androidTest/java/com/getcapacitor/myapp/ExampleInstrumentedTest.java

@@ -0,0 +1,26 @@
+package com.getcapacitor.myapp;
+
+import static org.junit.Assert.*;
+
+import android.content.Context;
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import androidx.test.platform.app.InstrumentationRegistry;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+/**
+ * Instrumented test, which will execute on an Android device.
+ *
+ * @see <a href="http://d.android.com/tools/testing">Testing documentation</a>
+ */
+@RunWith(AndroidJUnit4.class)
+public class ExampleInstrumentedTest {
+
+    @Test
+    public void useAppContext() throws Exception {
+        // Context of the app under test.
+        Context appContext = InstrumentationRegistry.getInstrumentation().getTargetContext();
+
+        assertEquals("com.getcapacitor.app", appContext.getPackageName());
+    }
+}