Add GitHub Actions workflow for pull request verification

[TyostoKarry]

Add a GitHub Actions workflow (verify-pr.yml) to automatically check code quality and build integrity on pull requests.

• Trigger on pull requests to main branch.
• Set up Java using actions/setup-java@v5
  • Use Temurin distribution with java-version: 21.
• Run Ktlint for code quality
• Run all tests to ensure build stability.
• Assemble project artifact with ./gradlew assemble
• Fail workflow if any step fails to prevent merging broken builds.