Add nodes to the canvas

## Context

Parent: #6897 — FE MQTT Migration

Migrate the `custom:add-nodes` action from HTTP to MQTT transport. This action allows the Expert to programmatically add nodes to the Node-RED editor canvas.

## Scope

- Implement `custom:add-nodes` over MQTT pub/sub
- Ensure nodes are not created/shown on top of one another (initial tests pass, but needs validation for complex flows)
- Handle acknowledgment/confirmation from the editor

## Estimate

1h

## Acceptance Criteria

- [ ] Expert can add nodes to the canvas via MQTT message
- [ ] Nodes are positioned correctly and don't overlap
- [ ] Works for single and multiple node additions