ci: add optional release signing from repository secrets

.gitea/workflows/android-build.yml

@@ -53,6 +53,39 @@ jobs:
             "platforms;android-35" \
             "build-tools;35.0.0"
 
+      - name: Prepare optional release signing
+        if: github.event_name == 'release'
+        env:
+          ANDROID_KEYSTORE_BASE64: ${{ secrets.ANDROID_KEYSTORE_BASE64 }}
+          ANDROID_KEYSTORE_PASSWORD: ${{ secrets.ANDROID_KEYSTORE_PASSWORD }}
+          ANDROID_KEY_ALIAS: ${{ secrets.ANDROID_KEY_ALIAS }}
+          ANDROID_KEY_PASSWORD: ${{ secrets.ANDROID_KEY_PASSWORD }}
+        shell: bash
+        run: |
+          set -euo pipefail
+          missing=()
+          for key in ANDROID_KEYSTORE_BASE64 ANDROID_KEYSTORE_PASSWORD ANDROID_KEY_ALIAS ANDROID_KEY_PASSWORD; do
+            if [ -z "${!key:-}" ]; then
+              missing+=("$key")
+            fi
+          done
+
+          if [ ${#missing[@]} -gt 0 ]; then
+            printf -v missing_csv '%s, ' "${missing[@]}"
+            missing_csv="${missing_csv%, }"
+            echo "::warning::Release signing secrets missing: ${missing_csv}. Building unsigned/default-signed release AAB."
+            exit 0
+          fi
+
+          mkdir -p .ci-secrets
+          printf '%s' "$ANDROID_KEYSTORE_BASE64" | base64 -d > .ci-secrets/release-keystore.jks
+          cat > keystore.properties <<EOF
+          storeFile=.ci-secrets/release-keystore.jks
+          storePassword=$ANDROID_KEYSTORE_PASSWORD
+          keyAlias=$ANDROID_KEY_ALIAS
+          keyPassword=$ANDROID_KEY_PASSWORD
+          EOF
+
       - name: Make Gradle wrapper executable
         run: chmod +x gradlew