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/Nodes/SyncNodeCommandHandlerTests.cs

@@ -55,7 +55,7 @@ public class SyncNodeCommandHandlerTests
         using var dbContext = InMemoryDbContextFactory.Create();
         var node = CreateNode();
         var inbound = Inbound.FromRemote(node.Id, "old-id", VpnProtocol.Vless, "old", 443);
-        inbound.Publish("Old", [Guid.NewGuid()]);
+        inbound.Publish("Old", null, [Guid.NewGuid()]);
         var config = VpnConfig.Create(Guid.NewGuid(), inbound.Id, VpnProtocol.Vless, null);
         config.AssignRemoteClient("external-id");