feat: add GitHub Actions workflow for release builds

- Build APK automatically when a release is published
- Upload APK as release asset
- Support custom keystore signing via GitHub secrets
- Fall back to debug signing when secrets not configured

.github/workflows/release.yml

@@ -0,0 +1,42 @@
+name: Build Release APK
+
+on:
+  release:
+    types: [published]
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          java-version: '17'
+          distribution: 'temurin'
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Decode keystore
+        if: ${{ env.KEYSTORE_BASE64 != '' }}
+        env:
+          KEYSTORE_BASE64: ${{ secrets.KEYSTORE_BASE64 }}
+        run: |
+          echo "$KEYSTORE_BASE64" | base64 -d > app/release.keystore
+
+      - name: Build Release APK
+        env:
+          KEYSTORE_BASE64: ${{ secrets.KEYSTORE_BASE64 }}
+          KEYSTORE_PASSWORD: ${{ secrets.KEYSTORE_PASSWORD }}
+          KEY_ALIAS: ${{ secrets.KEY_ALIAS }}
+          KEY_PASSWORD: ${{ secrets.KEY_PASSWORD }}
+        run: ./gradlew assembleRelease
+
+      - name: Upload APK to release
+        uses: softprops/action-gh-release@v2
+        with:
+          files: app/build/outputs/apk/release/app-release.apk