Add GitHub Actions CI workflow

- Builds release APK on push and pull requests
- Runs tests
- Uploads APK artifacts for 7 days
- Uses JDK 17 for Gradle 8.5 compatibility
- Checks out submodules recursively

Author: Max Lv

.github/workflows/build.yml

@@ -0,0 +1,48 @@
+name: Android CI
+
+on:
+  push:
+    branches: [ master, elastic-curran ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - name: Set up JDK 17
+      uses: actions/setup-java@v4
+      with:
+        java-version: '17'
+        distribution: 'temurin'
+        cache: gradle
+
+    - name: Grant execute permission for gradlew
+      run: chmod +x gradlew
+
+    - name: Build with Gradle
+      run: ./gradlew assembleRelease --stacktrace
+
+    - name: Run tests
+      run: ./gradlew test --stacktrace
+
+    - name: Upload APK
+      uses: actions/upload-artifact@v4
+      with:
+        name: app-release
+        path: app/build/outputs/apk/release/*.apk
+        retention-days: 7
+
+    - name: Build summary
+      run: |
+        echo "### Build Summary :rocket:" >> $GITHUB_STEP_SUMMARY
+        echo "" >> $GITHUB_STEP_SUMMARY
+        echo "- **Status**: ✅ Success" >> $GITHUB_STEP_SUMMARY
+        echo "- **APK Location**: \`app/build/outputs/apk/release/\`" >> $GITHUB_STEP_SUMMARY
+        ls -lh app/build/outputs/apk/release/*.apk >> $GITHUB_STEP_SUMMARY || true