v3.0.0 - MaglevCMS 3.0, Ruby 4.0.3, CI Tests

Changed

• Updated Ruby image to 4.0.3 (pinned patch version)
• Updated MaglevCMS to 3.0.1 (major version — new maglev:publish_site step required)
• Dropped sed workaround for maglev_client_javascript_tags (now the canonical helper in 3.x)

Added

• GitHub Actions CI workflow testing both flavors from scratch to browsable website
• Local just-rails-8/test.sh for end-to-end flavor testing