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.

Cargo.toml

@@ -16,3 +16,4 @@ tokio = { version = "1.49.0", features = ["full"] }
 tracing-subscriber = { version = "0.3.22", features = ["env-filter"] }
 md5 = "0.8.0"
 tower-http = {version = "0.6.8", features = ["trace"]}
+maud = { version = "0.27.0", features = ["axum"] }