Add GitHub Actions release workflow

[Vianpyro]

Closes #18

Changes

• Add .github/workflows/release.yml
• Triggers on release: created and builds the project and attaches the JAR to the existing release
• Excludes the -sources.jar from the upload
• Uses the same Java 25 + Gradle setup as the build workflow

How it works

1. You create a release via the GitHub UI as usual (I assume)
2. The workflow triggers, builds the project from the tagged commit
3. The built JAR (e.g. adaptiveview-2.4.3+26.1.jar) is automatically attached to the release
   No change to the existing release process: it just automates the JAR upload.

[Tre5et]

This workflow looks good. I will use it to create the next release.