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/Pages/Library.razor

@@ -2,16 +2,52 @@
 @attribute [Authorize]
 
 <div class="library-page">
-    <h1>Biblioteka</h1>
-    <p>Twoja kolekcja książek i dokumentów pojawi się tutaj wkrótce.</p>
+    <header class="library-header">
+        <h1>Biblioteka</h1>
+        <AuthorizeView Roles="Admin, ContentManager">
+            <NexusButton Class="add-book-trigger">
+                [+] Add New Book
+            </NexusButton>
+        </AuthorizeView>
+    </header>
+    
+    <div class="library-content glass-panel">
+        <div class="empty-state">
+            <p>Twoja kolekcja książek i dokumentów pojawi się tutaj wkrótce.</p>
+        </div>
+    </div>
 </div>
 
 <style>
     .library-page {
-        padding: 2rem;
+        padding: 3rem 2rem;
+        max-width: 1200px;
+        margin: 0 auto;
     }
+
+    .library-header {
+        display: flex;
+        justify-content: space-between;
+        align-items: center;
+        margin-bottom: 2.5rem;
+    }
+
     h1 {
-        margin-bottom: 1rem;
-        color: #fff;
+        font-family: var(--nexus-font-serif);
+        font-size: 2.5rem;
+        margin: 0;
+        color: var(--nexus-text);
+    }
+
+    .library-content {
+        min-height: 400px;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+    }
+
+    .empty-state {
+        text-align: center;
+        opacity: 0.6;
     }
 </style>