Add label-gated auto-release workflow for merged PRs

[Copilot]

This PR implements automated multi-platform release publishing from GitHub Actions. When a PR labeled release is merged into main, CI now builds release artifacts across project targets and publishes a GitHub Release using the repository version.

• Trigger + guardrails

  • Added .github/workflows/auto-release.yml on pull_request.closed.
  • Release pipeline runs only if:
    • PR is merged
    • base branch is main
    • PR has release label
  • Added workflow-level concurrency keyed by PR number.

• Version resolution

  • Reads release version from mobile/android/app/build.gradle.kts (versionName).
  • Falls back to desktop/pubspec.yaml (version:) if Android version is unavailable.
  • Emits explicit failure diagnostics when version extraction fails.

• Cross-platform build + artifact packaging

  • Reuses existing build patterns to produce:
    • API JAR
    • Web build archive (web-dist.zip)
    • Desktop macOS archive
    • Desktop Windows archive
    • Android release APK
  • Pins Flutter in release workflow (flutter-version: 3.35.x) for reproducibility.

• Release publication

  • Downloads all produced artifacts in a final release job.
  • Publishes GitHub Release via softprops/action-gh-release@v2 with:
    • tag v<version>
    • release name Release v<version>
    • generated release notes
    • attached build artifacts

• Docs

  • Updated README.md CI/workflow section to include auto-release.yml and describe its trigger condition.

if: >-
  github.event.pull_request.merged == true &&
  github.event.pull_request.base.ref == 'main' &&
  contains(github.event.pull_request.labels.*.name, 'release')