feat(ui/graph): Knowledge Graph Refinement and Sidebar Hierarchy (#25)

This PR addresses several UI/UX and architectural refinements for the Knowledge Graph and Intelligence Sidebar.

### Key Changes:
- **Knowledge Graph (#21, #22)**:
  - Implemented \"pill-shaped\" nodes with dynamic label truncation and SVG tooltips.
  - Added bound-constrained simulation to keep nodes within the viewport.
  - Integrated `ResizeObserver` for dynamic layout handling.
  - Implemented Zoom-to-Fit functionality.
  - Enforced unique concept IDs in AI prompts and hardened JS logic to prevent multi-selection bugs.
- **Intelligence Sidebar (#23)**:
  - Improved visual depth with a radial gradient background for the graph.
  - Increased sidebar divider contrast for better layering.
  - Transformed graph controls into a floating glassmorphism panel.
  - Relocated the \"Logout\" action to the toolbar bottom and rebranded it as \"Exit\".

Fixes #21
Fixes #22
Fixes #23

Reviewed-on: #25
Co-authored-by: Marek Jasiński <jasins.marek@gmail.com>
Co-committed-by: Marek Jasiński <jasins.marek@gmail.com>

src/NexusReader.UI.Shared/Components/Molecules/IntelligenceToolbar.razor

@@ -38,7 +38,7 @@
         <button class="toolbar-item" title="Global Settings">
             <NexusIcon Name="settings" Size="20" />
         </button>
-        <button class="toolbar-item logout-item" @onclick="HandleLogout" title="Logout">
+        <button class="toolbar-item logout-item" @onclick="HandleLogout" title="Exit">
             <NexusIcon Name="log-out" Size="20" />
         </button>
     </div>