ci: add manual release workflow

[carlosflorencio]

GitHub Actions could only publish these plugins after someone created and pushed a matching v* tag. This adds a manual release entry point so maintainers can select a patch, minor, or major bump in the Actions UI; the workflow updates the version metadata, commits and tags it, then reuses the existing cross-platform packaging and release job. Existing tag-triggered releases remain supported.

Validation

• Parsed the workflow YAML successfully.
• git diff --check passes.
• Verified the package filename and default branch used by this repository.

Checklist

• I have performed a self-review of my code.
• I have manually tested my changes and they work as expected.
• My changes have tests that cover the new functionality and edge cases.
• If my change touches UI files (apps/web/), I have added or updated Playwright e2e tests in apps/web/e2e/ and verified them with make test-e2e.
• I checked whether this affects public docs in docs/public/** and updated them or noted why no docs change is needed.

---

Summary by cubic

Adds a manual release workflow so maintainers can trigger patch/minor/major releases from GitHub Actions without pre-creating tags. It calculates the next version, updates metadata and changelog, commits and tags, then builds and publishes; tag-triggered releases still work.

• New Features
  • Adds workflow_dispatch with bump (patch/minor/major) and dry_run inputs.
  • Prepare job (on main) computes next SemVer from manifest.yaml and latest v* tag, updates manifest.yaml, Makefile, README.md, generates CHANGELOG.md, then commits and tags.
  • Publish job builds cross-platform package and creates the release via softprops/action-gh-release; runs for tag pushes and successful manual runs.

^{Written for commit d191fbd. Summary will update on new commits.}