[Feature Request]: Update server should be able to deliver critical messages to the desktop client

## Summary

When the auto-update mechanism itself is broken (as seen in issues like #9372), there is currently no way to inform affected users directly within the client. This creates a situation where users are silently stuck on an outdated version with no indication that manual intervention is required.

## Proposed Solution

Extend the update check response payload from the update server to include an optional `message` field. The desktop client would display this message prominently in the update dialog or as a system tray notification.

Example response:
```json
{
"version": "4.0.6",
"download": "https://...",
"message": "Auto-update is broken in versions 4.0.4 and 4.0.5. Please update manually."
}
```

This would allow the Nextcloud team to communicate critical issues — such as a broken updater, a security advisory, or a required manual step — directly to users without relying on them to check GitHub, forums, or release notes.

## Why This Matters

The auto-update failure in 4.0.4/4.0.5 is a good example of a "meta-problem": the very mechanism meant to fix issues is itself broken. In such cases, the only reliable communication channel left *is* the update server response — since the client still polls it even when it cannot act on the result.

Other applications (e.g., Firefox, VS Code) have shown that in-app messaging during update checks is an effective and low-effort way to keep users informed.

## Implementation Effort

Low: The client already parses the update server response. Adding support for an optional `message` field and displaying it in the UI would be a minimal change. The update server side would simply require a configurable field in its response.

## Related Issues
- #9372 (Auto-update fails silently on Windows)