Add support for right-click context menu

[philippjfr]

Description

Adds two features to panel-reactflow:

1. Node context menus

Adds support for per-node right-click context menus via a new Node.context_menu() method. Users override this method on their Node subclass to return any Panel component, which is rendered as a floating overlay at the click position.

from panel_reactflow import Node, ReactFlow
import panel as pn

class TaskNode(Node):
      def context_menu(self):
            return pn.Column(
                pn.widgets.Button(label="Run"),
                pn.widgets.Button(label="Delete"),
            )

flow = ReactFlow(nodes=[TaskNode(id="t1", position={"x": 0, "y": 0})])

• Menu appears on right-click, dismissed on pane click
• Only works for Node subclass instances (dict nodes are ignored)
• Returning None disables the menu (default behavior)
• Emits a "node_context_menu" event with node_id and position

2. Editor rendering optimization

Previously, all node and edge editor views were serialized and sent to the frontend regardless of whether they were visible. This is wasteful, especially in "side" mode where only one editor is shown at a time.

Now:

• Side mode: Only the selected node/edge editor is sent via a new _selected_editor Child parameter (instead of N editors)
• Node/toolbar mode: Node editors still sent per-node (all are needed), but edge editors are no longer sent (they were never rendered in these modes)
• Removed dead JS code (edgeEditorMap, nodeEditorMap, edgeEditors prop on FlowInner)

AI Disclosure

• Tool & Model: Claude Code + Opus 4.6
• Usage: Implementation of both features including Python, JSX, CSS, tests, documentation, and examples.
• I have tested all AI-generated content in my PR.
• I take responsibility for all AI-generated content in my PR.

Checklist

• Tests added and are passing
• Added documentation