const ALLOWED_DOMAIN_SUFFIX = '.azure-devices.net';

This blocks all Azure China (21Vianet) IoT Hubs, which use .azure-devices.cn as their domain suffix, as documented here:
https://learn.microsoft.com/en-us/azure/iot-hub/iot-hub-devguide-endpoints

To Reproduce
1. Open Azure IoT Explorer v0.15.15 or v0.15.16
2. Click "Add connection"
3. Paste a connection string with .azure-devices.cn hostname
4. See error: "Invalid hostname: must be a valid Azure IoT Hub endpoint (*.azure-devices.net)"

Expected behavior
Azure IoT Explorer should support both:
- Global Azure: *.azure-devices.net
- Azure China (21Vianet): *.azure-devices.cn

Suggested fix

Update ALLOWED_DOMAIN_SUFFIX in public/handlers/urlValidator.ts to also accept .azure-devices.cn, or refactor to use an array of allowed
suffixes:

const ALLOWED_DOMAIN_SUFFIXES = ['.azure-devices.net', '.azure-devices.cn'];

Desktop:
- OS: Windows 11
- App version: 0.15.15 (also confirmed in 0.15.16)

Additional context
- Related PR that introduced the regression: #720
- Related PR #726 updated EventHub validation but did not address IoT Hub hostname validation
- Issue #453 previously reported Azure China connectivity (2021) but that was a different error — the hostname validation did not exist
at that time