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.lock

@@ -455,6 +455,30 @@ version = "0.8.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "47e1ffaa40ddd1f3ed91f717a33c8c0ee23fff369e3aa8772b9605cc1d22f4c3"
 
+[[package]]
+name = "maud"
+version = "0.27.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8156733e27020ea5c684db5beac5d1d611e1272ab17901a49466294b84fc217e"
+dependencies = [
+ "axum-core",
+ "http",
+ "itoa",
+ "maud_macros",
+]
+
+[[package]]
+name = "maud_macros"
+version = "0.27.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7261b00f3952f617899bc012e3dbd56e4f0110a038175929fa5d18e5a19913ca"
+dependencies = [
+ "proc-macro2",
+ "proc-macro2-diagnostics",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "md5"
 version = "0.8.0"
@@ -567,6 +591,18 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "proc-macro2-diagnostics"
+version = "0.10.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "af066a9c399a26e020ada66a034357a868728e72cd426f3adcd35f80d88d88c8"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "version_check",
+]
+
 [[package]]
 name = "quote"
 version = "1.0.44"
@@ -770,6 +806,7 @@ dependencies = [
  "axum",
  "diesel",
  "dotenvy",
+ "maud",
  "md5",
  "serde",
  "serde_json",
@@ -1013,6 +1050,12 @@ version = "0.2.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "accd4ea62f7bb7a82fe23066fb0957d48ef677f6eeb8215f372f52e48bb32426"
 
+[[package]]
+name = "version_check"
+version = "0.9.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0b928f33d975fc6ad9f86c8f283853ad26bdd5b10b7f1542aa2fa15e2289105a"
+
 [[package]]
 name = "wasi"
 version = "0.11.1+wasi-snapshot-preview1"