Refresh the Hive login and dashboard shell with a Tuist-style design

Redesign the login page to match the Tuist style, with a centered card, product logo, and blurred purple gradient background. The new dashboard shell has a header bar, sidebar, and content area, and the Hive logo is used in the header, login frame, and favicon. Under the hood, CSS is split into route, layout, and component files and uses a data-part pattern instead of BEM-style classes for easier maintenance.