feat: Use GitHub auto-generated release notes instead of single commit message (#724)

## ℹ️ Description
Currently, release changelogs only show the last commit message, which
doesn't provide sufficient visibility into all changes included in a
release. This PR improves the release workflow to use GitHub's
auto-generated release notes, providing a comprehensive changelog of all
commits and PRs since the previous release.

- Addresses the issue of insufficient release changelog detail
- Improves transparency for users reviewing what changed in each release

## 📋 Changes Summary

- Added `--generate-notes` flag to `gh release create` command in
`.github/workflows/build.yml`
- Renamed `COMMIT_MSG` environment variable to `LEGAL_NOTICE` for better
clarity
- Legal disclaimers now append after the auto-generated changelog
instead of replacing it
- The auto-generated notes will include:
  - All commits since the last release
  - All merged PRs since the last release
  - Contributor attribution
  - Automatic categorization (New Contributors, Full Changelog, etc.)

### ⚙️ Type of Change
Select the type(s) of change(s) included in this pull request:
- [ ] 🐞 Bug fix (non-breaking change which fixes an issue)
- [x] ✨ New feature (adds new functionality without breaking existing
usage)
- [ ] 💥 Breaking change (changes that might break existing user setups,
scripts, or configurations)


## ✅ Checklist
Before requesting a review, confirm the following:
- [x] I have reviewed my changes to ensure they meet the project's
standards.
- [x] I have tested my changes and ensured that all tests pass (`pdm run
test`).
- [x] I have formatted the code (`pdm run format`).
- [x] I have verified that linting passes (`pdm run lint`).
- [x] I have updated documentation where necessary.

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
* Release process updated to embed a bilingual (English/German) legal
notice directly into generated release notes.
* Release creation now auto-generates notes using that legal notice so
published releases consistently include the legal text.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.github/workflows/build.yml

@@ -451,9 +451,7 @@ jobs:
       env:
         GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         RELEASE_NAME: ${{ steps.release.outputs.name }}
-        COMMIT_MSG: |  # https://stackoverflow.com/a/78420438/5116073
-          ${{ github.event.head_commit.message }}
-
+        LEGAL_NOTICE: |
           ---
           #### ⚠️ Rechtlicher Hinweis
           <p>Die Verwendung dieses Programms kann unter Umständen gegen die zum jeweiligen Zeitpunkt bei kleinanzeigen.de geltenden Nutzungsbedingungen verstoßen.
@@ -473,7 +471,8 @@ jobs:
           --title "$RELEASE_NAME" \
           ${{ steps.release.outputs.name == 'latest' && '--latest' || '' }} \
           ${{ steps.release.outputs.name == 'preview' && '--prerelease' || '' }} \
-          --notes "$COMMIT_MSG" \
+          --generate-notes \
+          --notes "$LEGAL_NOTICE" \
           --target "${{ github.sha }}" \
           kleinanzeigen-bot-darwin-amd64 \
           kleinanzeigen-bot-darwin-arm64 \