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/Create/CreateVpnConfigCommandHandlerTests.cs

@@ -52,7 +52,7 @@ public class CreateVpnConfigCommandHandlerTests
             null
         );
         var inbound = Inbound.FromRemote(node.Id, "1", VpnProtocol.Vless, "remark", 443);
-        inbound.Publish(null, [roleId]);
+        inbound.Publish(null, null, [roleId]);
 
         dbContext.Nodes.Add(node);
         dbContext.Inbounds.Add(inbound);