Add user feedback buttons with PostHog integration

## Overview
Add thumbs up/down feedback buttons to collect user sentiment on FlowFuse expert responses and send data to PostHog for analysis.

## Requirements

### UI Component
- Add thumbs up 👍 / thumbs down 👎 buttons to the interface (see screenshot in Slack)
- Buttons should appear after FlowFuse expert responses

### PostHog Integration
- Send feedback events to PostHog when users click buttons
- Include in the event payload:
- User sentiment (positive/negative)
- Conversation history up to that point including plain text responses to understand what the user didn't/did like

### Technical Considerations
- PostHog payload sizes and traffic should be acceptable for conversation history
- Estimated effort: 0.5 day (per Serban)

## Context
From Slack discussion: Users need a way to provide quick feedback on Claude responses. This data will help improve the product by understanding what responses work well and which don't.

## References
- Slack conversation: https://flowfuse.slack.com/archives/C09QS57R8AD/p1769428483403809