ci(android): add debug APK build job

[priyanshrv1-oss]

Description

This PR adds a GitHub Actions job to build debug APKs for the Android app and fixes a Gradle configuration crash that prevented builds in environments without a release keystore.

Changes

• opennow-stable/android/app/build.gradle

  • Moved apply plugin statements before android block
  • Wrapped keystore loading in if (keystorePropertiesFile.exists()) conditional
  • Changed keystore path from hardcoded Windows path to rootProject.file("keystore.properties")
  • Release signing config now optional; builds proceed without keystore

• .github/workflows/auto-build.yml

  • Added build-android job running on ubuntu-latest
  • Set up Node.js 22, Java 21, and npm caching
  • Install npm dependencies, build web assets, and sync Capacitor Android project
  • Install Android SDK platform 36 and build-tools 35.0.0 via sdkmanager
  • Create android/local.properties with SDK path
  • Run ./gradlew assembleDebug and upload APK artifact as opennow-android-debug
  • Job runs independently of existing Electron build matrix