[Feature] Add graph export as PNG/image

[d-oit]

Description

There is currently no way to export the knowledge graph visualization as an image. Users should be able to save the graph as a PNG for sharing, documentation, or presentations.

Proposed Implementation

1. Use Sigma.js toCanvas() method to render the graph to a canvas
2. Convert canvas to PNG via canvas.toDataURL()
3. Add "Export as PNG" button in GraphControls.tsx
4. Support custom resolution/DPI for high-quality exports
5. Include legend and title in the exported image

Acceptance Criteria

• "Export as PNG" button in graph controls
• Exported image captures the current graph view
• High-DPI support for retina displays
• Optional legend overlay

[github-actions]

👋 This looks like a duplicate of #189.

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