chore: Back-port 5.4.2 fixes to v6 (#298, #302, #303, #304)

[rickbutterfield]

Back-ports the four fixes merged into v5/dev onto v6/dev, ahead of a 6.0.1 patch release. Done by cherry-picking each PR merge commit (-m 1).

• fix: resolve nodeKey for previews nested in block lists #298 — nested Block List nodeKey resolution (+ web-test-runner unit tests)
• test: Migrate acceptance tests to @umbraco-cms/acceptance-test-helpers #304 — migrate acceptance tests to @umbraco-cms/acceptance-test-helpers (resolved to ^18.0.0 to match v6's @umbraco-cms/backoffice@18.0.0)
• ci: Build frontend assets before packing in release workflow #303 — build the frontend in the release workflow
• fix: Render RTE block previews nested in a side-panel modal (#300) #302 — render RTE block previews nested in a side-panel modal

Conflict resolutions

• version.json untouched → stays 6.1.0-alpha.
• UI package.json: kept v6's @hey-api/openapi-ts@0.97.1 / @umbraco-cms/backoffice@18.0.0, added fix: resolve nodeKey for previews nested in block lists #298's test-infra devDeps.
• Acceptance package.json: kept @umbraco/json-models-builders@^18.0.2, migrated helper to @umbraco-cms/acceptance-test-helpers@^18.0.0.
• Rebuilt the frontend bundle from merged source; regenerated both lockfiles.

Verification

• UI unit tests (web-test-runner): 8 passed (the nodeKey-resolution suite runs against backoffice 18).
• npm run build (tsc + vite): clean.
• dotnet build of the package + test site: 0 errors.

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