Implement node selection state management

[ChristianHuehn]

Feature request

Part of Epic #136

Description

As a developer, I want to implement node selection state management using RxJS so that the application can track which node is selected and update the panel accordingly.

Acceptance criteria

• Service created to manage node selection state using RxJS
• Observable stream for selected node changes
• Click handler on graph nodes updates the selection state
• Panel component subscribes to selection changes
• State is properly cleaned up on component destruction
• Clicking a node while panel is open updates the selected node

Assumptions & Exclusions

• Uses RxJS BehaviorSubject or similar for state management
• No persistence of selected node (resets on page refresh)
• Single node selection only (no multi-select)

Development notes (optional Task Breakdown)

• Create NodeSelectionService with RxJS state management
• Add click handler to graph nodes in visualization component
• Implement observable stream for selected node
• Connect panel component to selection observable
• Handle edge cases (node deletion, graph reload)
• Add proper cleanup in ngOnDestroy

Open questions

None - reactive state management pattern is well-defined.