feat(ui/auth): implement authorized 'Add Book' trigger in Library view #32 (#38)

Resolves #32

### Changes:
- Added '[+] Add New Book' button to `Library.razor`.
- Wrapped the button in `AuthorizeView` for `Admin` and `ContentManager` roles.
- Updated `NexusButton` styling to follow standards: secondary gray background with neon glow on hover/focus.
- Refined `Library.razor` layout with a header and glassmorphism panel.

---------

Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Reviewed-on: #38
Reviewed-by: Marek Jaisński <jasins.marek@gmail.com>
Co-authored-by: Antigravity <antigravity@google.com>
Co-committed-by: Antigravity <antigravity@google.com>

src/NexusReader.UI.Shared/Components/Atoms/NexusButton.razor.css

@@ -4,32 +4,35 @@
     justify-content: center;
     min-width: 44px;
     min-height: 44px;
-    padding: 0.5rem 1rem;
-    background-color: var(--nexus-card);
-    color: var(--nexus-neon);
-    border: 1px solid var(--nexus-neon);
+    padding: 0.5rem 1.25rem;
+    background-color: rgba(255, 255, 255, 0.05);
+    color: #ffffff;
+    border: 1px solid rgba(255, 255, 255, 0.1);
     font-family: var(--nexus-font-sans);
     font-weight: 500;
-    font-size: 1rem;
-    border-radius: 8px;
+    font-size: 0.95rem;
+    border-radius: 10px;
     cursor: pointer;
-    transition: all 0.2s ease;
-    box-shadow: 0 0 5px rgba(0, 255, 153, 0.1);
+    transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
+    box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1);
 }
 
-.nexus-btn:hover:not(:disabled) {
-    background-color: rgba(0, 255, 153, 0.1);
+.nexus-btn:hover:not(:disabled),
+.nexus-btn:focus:not(:disabled) {
+    background-color: rgba(255, 255, 255, 0.08);
+    border-color: rgba(0, 255, 153, 0.4);
+    color: var(--nexus-neon);
     box-shadow: 0 0 15px rgba(0, 255, 153, 0.3);
+    outline: none;
+    transform: translateY(-1px);
 }
 
 .nexus-btn:active:not(:disabled) {
-    transform: scale(0.98);
+    transform: translateY(0) scale(0.98);
 }
 
 .nexus-btn:disabled {
-    opacity: 0.5;
+    opacity: 0.4;
     cursor: not-allowed;
-    border-color: #555;
-    color: #555;
-    box-shadow: none;
+    filter: grayscale(1);
 }