Read live canvas state

## Context

Parent: #6897 — FE MQTT Migration

Implement the `custom:get-canvas-flows` action over MQTT/WebSocket. This action reads the live canvas state (current flows, nodes, wires, tabs) from the Node-RED editor.

**This action was previously blocked** — it requires the WebSocket-based MQTT migration to be complete before it can be implemented, as it needs real-time bidirectional communication for state sync.

## Scope

- Implement `custom:get-canvas-flows` over MQTT
- Return current editor state: all nodes, wires, tabs, subflows
- Enable the Expert to have context-aware understanding of what the user has built
- This is critical for the Expert's ability to provide relevant suggestions based on the current flow state

## Estimate

3h

## Acceptance Criteria

- [ ] Expert can request and receive the current canvas state via MQTT
- [ ] Response includes all nodes, wires, tabs, and subflows
- [ ] State is accurate and reflects unsaved changes (editor state, not just deployed state)
- [ ] Response handles large flows without truncation