Add load balancer marker for inbound country flag

Reserved CountryCode "XX" (Inbound.BalancerCountryCode) marks an inbound
behind a load balancer whose exit country is unknown: the site shows a
Shuffle icon with a tooltip instead of a flag, the bot shows 🔀. Admin can
pick it in the publish dialog. Fits the existing column, no migration.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/PnvPanel.Domain.Tests/Inbounds/InboundTests.cs

@@ -51,6 +51,7 @@ public class InboundTests
     [Theory]
     [InlineData("de", "DE")]
     [InlineData(" NL ", "NL")]
+    [InlineData("xx", Inbound.BalancerCountryCode)]
     [InlineData("", null)]
     [InlineData(null, null)]
     public void Publish_NormalizesCountryCode(string? input, string? expected)