Add interactive Adjacency Matrix Viewer

New docs/matrix.html page with:
- Load JSON graphs (drag-and-drop or file picker)
- Interactive matrix with hover tooltips showing source→target, weight, type
- 5 sort modes: alphabetical, degree (asc/desc), community cluster, original
- 3 color modes: edge weight heatmap, binary, edge type
- Community detection via label propagation
- Stats bar: nodes, edges, density, max weight, communities
- Export as PNG or CSV
- Sample graph generator for quick testing
- Toggle value labels and force-symmetric options
- Sidebar links added to index.html and stats.html

Usage: Open docs/matrix.html in browser, load a JSON graph file
(same format as Graph Explorer), explore the adjacency structure.

Author: sauravbhattacharya001

docs/index.html

@@ -28,6 +28,7 @@ <h2>GraphVisual</h2>
         <a href="query.html" class="sidebar-link"><span class="icon">🔎</span>Query Playground</a>
         <a href="generator.html" class="sidebar-link"><span class="icon">🎲</span>Graph Generator</a>
         <a href="stats.html" class="sidebar-link"><span class="icon">📈</span>Statistics</a>
+        <a href="matrix.html" class="sidebar-link"><span class="icon">🔢</span>Adjacency Matrix</a>
     </div>
     <div class="sidebar-section">
         <div class="sidebar-section-title">Architecture</div>