Explore MQTT pubsub for FE Expert

## Context

Parent: #6897 — FE MQTT Migration

Exploration task to evaluate MQTT client libraries and connection patterns for the Expert frontend. Reference the existing device Node-RED logs component which already pulls WS over MQTT for the frontend.

## Scope

- Investigate MQTT-over-WebSocket client libraries suitable for the FlowFuse Vue frontend
- Study the device Node-RED logs component for reference on how WS over MQTT is pulled for the frontend
- Prototype basic subscribe/publish to EMQX broker from the FE
- Evaluate reconnection handling, message ordering, and QoS levels needed

## Estimate

2h

## Acceptance Criteria

- [ ] MQTT client library selected and justified
- [ ] Basic pub/sub prototype connecting to EMQX from the FE works
- [ ] Reconnection strategy documented
- [ ] Reference patterns from instance logs component noted