- Add Fund Session button to Matrix UI overlay (green plus icon)
- Create modal with three explanatory sections:
* What are Sats? - explains satoshis as smallest Bitcoin unit
* Why Fund Your Session? - explains how sats power Workshop AI agents
* Approximate Costs - shows cost ranges for different interaction types
- Add input field for funding amount (min 100 sats)
- Style modal with green theme to match Lightning/sats concept
- Add proper keyboard support (Enter to submit, Escape to close)
- Mobile-responsive design
Fixes#753
