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/Configs/GetMyConfigs/GetMyConfigsQueryHandlerTests.cs

@@ -22,7 +22,7 @@ public class GetMyConfigsQueryHandlerTests
         var otherUserId = Guid.NewGuid();
 
         var inbound = Inbound.FromRemote(Guid.NewGuid(), "1", VpnProtocol.Vless, "remark", 443);
-        inbound.Publish("My inbound", []);
+        inbound.Publish("My inbound", null, []);
 
         var activeConfig = VpnConfig.Create(userId, inbound.Id, VpnProtocol.Vless, "Active");
         var revokedConfig = VpnConfig.Create(userId, inbound.Id, VpnProtocol.Vless, "Revoked");