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/Configs/ListAllConfigsQueryHandlerTests.cs

@@ -26,7 +26,7 @@ public class ListAllConfigsQueryHandlerTests
             null
         );
         var inbound = Inbound.FromRemote(node.Id, "1", VpnProtocol.Vless, "remark", 443);
-        inbound.Publish("Germany", []);
+        inbound.Publish("Germany", null, []);
         var config = VpnConfig.Create(userId, inbound.Id, VpnProtocol.Vless, "my-config");
 
         dbContext.Nodes.Add(node);