Add inbound country flags and node load in location picker

- Inbound.CountryCode (ISO alpha-2), set on publish; flag shown before
  the location name (SVG via country-flag-icons on the site, emoji in bot)
- Node.Capacity (optional, not a limit); available inbounds report load
  percent of capacity or active config count, sorted least loaded first

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

backend/tests/PnvPanel.Application.Tests/Admin/Inbounds/PublishInboundCommandValidatorTests.cs

@@ -0,0 +1,35 @@
+using PnvPanel.Application.Admin.Inbounds;
+using Xunit;
+
+namespace PnvPanel.Application.Tests.Admin.Inbounds;
+
+public class PublishInboundCommandValidatorTests
+{
+    private readonly PublishInboundCommandValidator _validator = new();
+
+    private static PublishInboundCommand Command(string? countryCode) =>
+        new(Guid.NewGuid(), true, "EU Fast", countryCode, []);
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("DE")]
+    [InlineData("nl")]
+    public void Validate_WithEmptyOrAlpha2CountryCode_IsValid(string? countryCode)
+    {
+        var result = _validator.Validate(Command(countryCode));
+
+        Assert.True(result.IsValid);
+    }
+
+    [Theory]
+    [InlineData("DEU")]
+    [InlineData("D")]
+    [InlineData("1E")]
+    public void Validate_WithInvalidCountryCode_IsInvalid(string countryCode)
+    {
+        var result = _validator.Validate(Command(countryCode));
+
+        Assert.False(result.IsValid);
+    }
+}