Node wrapper LiveComponent for LiveFlow.

Wraps node content and handles positioning, selection state, and handle rendering. Supports three ways to customize node content:

1. Function component in node_types — simplest, receives @node assign
2. LiveComponent module in node_types — for stateful custom nodes
3. node_renderer fallback — global function for all unmatched types

Priority: type-specific (node_types) > node_renderer > default label.