Document that SPICE clipboard on Windows ARM requires virtio-ramfb (display backend dependency)

## Summary

Clipboard sharing via SPICE on Windows ARM VMs depends on the selected display backend, but this dependency is not documented and is non-obvious.

## Environment

- UTM version: 4.7.5 (118)
- macOS: 26.4.1 (25E253)
- Guest: Windows 11 ARM
- VM type: Virtualize

## Observations

With SPICE guest tools installed (`spice-vdagent` running):

Clipboard behavior varies depending on the display device:

| Display Device | Clipboard |
|----------------------|----------|
| virtio-ramfb | ✅ Works |
| virtio-ramfb-gl | ❌ Does not work |
| virtio-gpu / gpu-gl | ❌ Does not work |

## Key Insight

Clipboard integration requires:
1. SPICE guest agent (installed and running)
2. A SPICE-compatible display backend

Using accelerated/GL display modes appears to bypass or disable the SPICE channel required for clipboard integration, even though the agent is running and UTM UI indicates clipboard sharing is enabled.

## Why this is confusing

- Clipboard sharing can be enabled in UTM settings regardless of display backend
- SPICE services appear healthy inside the guest
- Failure mode is silent (no error, clipboard just doesn't work)

This makes it difficult to diagnose and can lead to incorrect assumptions that SPICE is unsupported on Windows ARM.

## Suggestion

Add documentation (and possibly UI hints) clarifying:

- Clipboard sharing requires both:
- SPICE guest agent
- A SPICE-compatible display backend (e.g. `virtio-ramfb`)
- GPU/GL display modes may disable clipboard integration

Optional: warn in UI when clipboard sharing is enabled with a non-SPICE-compatible display.

## Additional note

This behavior is reproducible and switching to `virtio-ramfb` immediately restores clipboard functionality.

Working settings: