Feat: Implement Admin Dashboard UI

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.

src/admin/mod.rs

@@ -0,0 +1,3 @@
+pub mod layout;
+pub mod routes;
+pub mod views;