uSync.Forms repo cleanup

Repo: KevinJump/uSync.Forms

Cleaned up GitHub branches — deleted 4 stale/superseded ones (master, dev/folders, v13/html-message-fix, v17/rc2-build), left one {version}/main branch per Umbraco major
Cleaned up Dependabot: dismissed 16 stale alerts pointing at a deleted pre-v9 project, added .github/dependabot.yml scoped to live projects, set it to ignore major-version bumps for Umbraco packages (this branch targets Umbraco 17 specifically)
Fixed 11 of 18 npm audit vulnerabilities via lockfile updates (postcss, rollup, vite, ws, nanoid, picomatch, uuid, markdown-it); pinned @umbraco-cms/backoffice to the 17.4.2 release to match the nuget package versions
Brought the repo up to the same baseline as uSync: added LICENSE (MPL-2.0), CODE_OF_CONDUCT.md, SECURITY.md, CHANGELOG.md, issue/PR templates, packages.lock.json for all three projects, and GitHub Actions workflows for CodeQL, PR builds, package builds, and tag-triggered NuGet releases