Add CI workflow for pull requests

[jvsena42]

Summary

Add a GitHub Actions CI workflow that runs on pull requests to automate code quality checks and builds.

Changes

• Add .github/workflows/ci.yml with a check job that runs on PRs (skips drafts)
• Configures JDK 17 + Gradle caching via official actions
• Runs Detekt, shared module compilation, shared unit tests, and Android debug APK build
• Adds concurrency control to cancel in-progress runs on new pushes

Test plan

• Open a PR and verify the CI workflow triggers
• Push a new commit to the PR and verify the previous run is cancelled
• Mark a PR as draft and verify the workflow is skipped

Checklist

• ./gradlew detekt passes
• Tested on Android
• Tested on iOS
• Updated documentation (if applicable)