feat: add git-cliff changelog generation to release workflow

[pfeerick]

Summary

• Adds a new generate-changelog job to build_release.yml that installs git-cliff and generates a changelog from the previous latest tag to HEAD
• Both the upload (Latest Themes Package) and upload-individual (Individual Theme Packages) release jobs now receive the changelog as their release body, so GitHub releases show what changed since the last push
• Adds cliff.toml at the repo root configuring conventional-commit parsing with groups suited to this repo: Features, Bug Fixes, Documentation, CI/CD, Chores, and Other

How it works

1. Before the latest tag is force-pushed, the job resolves the commit SHA it currently points to
2. git-cliff generates a changelog over the range <prev-sha>..HEAD (full history on first run when no latest tag exists yet)
3. The changelog is passed via job outputs to both upload jobs and injected as the body field on softprops/action-gh-release

Reviewer notes

• The generate-changelog job is gated with if: github.event_name != 'pull_request' — it won't run on PRs
• git-cliff is fetched at runtime (latest release) using the same wget pattern as EdgeTX/edgetx's release-drafter.yml
• Conventional commits in this repo already include PR numbers (e.g. feat: add theme (#93)) so they appear naturally in the changelog body
• Commits that don't match any conventional-commit prefix fall into the 📦 Other group rather than being dropped

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