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.Domain.Tests/Nodes/NodeTests.cs

@@ -212,4 +212,26 @@ public class NodeTests
         Assert.DoesNotContain("protected-secret", text);
         Assert.Contains("REDACTED", text);
     }
+
+    [Theory]
+    [InlineData(50)]
+    [InlineData(null)]
+    public void SetCapacity_WithPositiveOrNull_SetsCapacity(int? capacity)
+    {
+        var node = Node.Register("Germany-1", new Uri("https://de1.example.com"), Credentials, null);
+
+        node.SetCapacity(capacity);
+
+        Assert.Equal(capacity, node.Capacity);
+    }
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-1)]
+    public void SetCapacity_WithNonPositive_Throws(int capacity)
+    {
+        var node = Node.Register("Germany-1", new Uri("https://de1.example.com"), Credentials, null);
+
+        Assert.Throws<DomainException>(() => node.SetCapacity(capacity));
+    }
 }