Bulk import a flow

## Context

Parent: #6897 — FE MQTT Migration

Migrate the `custom:import-flow` action from HTTP to MQTT transport. This action allows the Expert to bulk import a complete flow (nodes + tabs + wires + subflows) into the Node-RED editor.

This is the primary mechanism for the Prompt-to-Prototype Pipeline (FlowFuse/product#45) — the Expert generates flow JSON and inserts it into the editor workspace.

## Scope

- Implement `custom:import-flow` over MQTT
- Need to investigate subflow mechanism of import — how subflows are handled during bulk import
- Validate that imported flows render correctly with proper layout

Related: FlowFuse/nr-assistant#172 — Support for Import Flow action

## Estimate

3h

## Acceptance Criteria

- [ ] Expert can bulk import a complete flow (nodes, tabs, wires) via MQTT message
- [ ] Subflows are correctly imported and resolved
- [ ] Imported flow renders with proper layout (nodes not overlapping)
- [ ] Large flows (20+ nodes) import without timeout or truncation