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/src/PnvPanel.Domain/Inbounds/Inbound.cs

@@ -18,6 +18,10 @@ public sealed class Inbound : Entity
     public bool IsPublished { get; private set; }
     public bool IsAvailable { get; private set; } = true;
     public string? DisplayName { get; private set; }
+
+    /// <summary>ISO 3166-1 alpha-2 (верхний регистр) — страна, флаг которой пользователь видит перед
+    /// именем инбаунда. Только для отображения, на подключение не влияет.</summary>
+    public string? CountryCode { get; private set; }
     public IReadOnlyList<Guid> AllowedRoleIds { get; private set; } = [];
     public DateTimeOffset? LastSyncAt { get; private set; }
 
@@ -57,11 +61,18 @@ public sealed class Inbound : Entity
         IsAvailable = true;
     }
 
-    public void Publish(string? displayName, IReadOnlyCollection<Guid> allowedRoleIds)
+    public void Publish(
+        string? displayName,
+        string? countryCode,
+        IReadOnlyCollection<Guid> allowedRoleIds
+    )
     {
         IsPublished = true;
         IsAvailable = true;
         DisplayName = displayName;
+        CountryCode = string.IsNullOrWhiteSpace(countryCode)
+            ? null
+            : countryCode.Trim().ToUpperInvariant();
         AllowedRoleIds = allowedRoleIds.Distinct().ToList();
     }