Add adjacency matrix heatmap visualization

New AdjacencyMatrixHeatmap panel displays the graph as a color-coded
matrix where cells represent edges, colored by type (friend, classmate,
familiar stranger, stranger, study group).

Features:
- Sort nodes by degree, name, or community (BFS ordering)
- Zoom with scroll wheel, pan with right-drag
- Hover tooltips showing node pair and edge details
- Row/column highlighting on hover
- Export to PNG
- Legend with edge type colors
- Diagonal cells show node degree as brightness

Access via 'Adjacency Matrix' button in the Tools panel.

Author: sauravbhattacharya001