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/Inbounds/PublishInboundCommandHandlerTests.cs

@@ -22,7 +22,7 @@ public class PublishInboundCommandHandlerTests
         var roleId = Guid.NewGuid();
         var handler = new PublishInboundCommandHandler(dbContext, _currentUser);
 
-        var command = new PublishInboundCommand(inbound.Id, true, "EU Fast", [roleId]);
+        var command = new PublishInboundCommand(inbound.Id, true, "EU Fast", "de", [roleId]);
 
         var result = await handler.Handle(command, CancellationToken.None);
 
@@ -31,6 +31,8 @@ public class PublishInboundCommandHandlerTests
         Assert.Equal("EU Fast", inbound.DisplayName);
         Assert.Contains(roleId, inbound.AllowedRoleIds);
         Assert.Equal("EU Fast", result.Value.DisplayName);
+        Assert.Equal("DE", inbound.CountryCode);
+        Assert.Equal("DE", result.Value.CountryCode);
     }
 
     [Fact]
@@ -38,13 +40,13 @@ public class PublishInboundCommandHandlerTests
     {
         using var dbContext = InMemoryDbContextFactory.Create();
         var inbound = Inbound.FromRemote(Guid.NewGuid(), "1", VpnProtocol.Vless, "remark", 443);
-        inbound.Publish("EU Fast", [Guid.NewGuid()]);
+        inbound.Publish("EU Fast", null, [Guid.NewGuid()]);
         dbContext.Inbounds.Add(inbound);
         await dbContext.SaveChangesAsync(CancellationToken.None);
 
         var handler = new PublishInboundCommandHandler(dbContext, _currentUser);
 
-        var command = new PublishInboundCommand(inbound.Id, false, null, []);
+        var command = new PublishInboundCommand(inbound.Id, false, null, null, []);
 
         var result = await handler.Handle(command, CancellationToken.None);
 
@@ -59,7 +61,7 @@ public class PublishInboundCommandHandlerTests
 
         var handler = new PublishInboundCommandHandler(dbContext, _currentUser);
 
-        var command = new PublishInboundCommand(Guid.NewGuid(), true, "EU Fast", []);
+        var command = new PublishInboundCommand(Guid.NewGuid(), true, "EU Fast", null, []);
 
         var result = await handler.Handle(command, CancellationToken.None);