New Atlas-style login shell and dashboard layout

The dashboard and login pages now use a new app shell inspired by the Atlas/Noora design: a header bar, sidebar, content slot, and a centered login frame with the Hive honeycomb logo, Tuist-style purple gradient background, and a clean call to action. This includes a new Layouts module, the Hive logo, and updated styles split across route, layout, and component CSS files. (https://github.com/tuist/hive/pull/1, https://raw.githubusercontent.com/tuist/hive/server@0.2.0/lib/hive_web/controllers/page_html.ex, https://raw.githubusercontent.com/tuist/hive/server@0.2.0/AGENTS.md, https://github.com/tuist/hive/releases/tag/server%400.2.0)