This commit introduces the foundational UI for the admin dashboard. It
includes:
- A new `maud`-based layout component for consistent page structure.
- A navigation sidebar and header.
- Basic views for dashboard overview, user management, library settings,
and about page.
- Integration with Tailwind CSS and Phosphor Icons for styling.
- Added `maud` and `axum-core` as dependencies.
3f33bcdf32