Fix Windows channel tree ordering

clients/windows/VoiceCat.Windows/ChannelHierarchy.cs

@@ -0,0 +1,32 @@
+namespace VoiceCat.Windows;
+
+/// <summary>A channel and its ordered children as displayed by the Windows client.</summary>
+public sealed record ChannelHierarchyItem(
+    ChannelInfo Channel,
+    IReadOnlyList<ChannelHierarchyItem> Children);
+
+public static class ChannelHierarchy
+{
+    /// <summary>
+    /// Builds the server-defined channel hierarchy. SortOrder orders siblings; LINQ's stable
+    /// ordering preserves the server's sequence when siblings have the same value.
+    /// </summary>
+    public static IReadOnlyList<ChannelHierarchyItem> Build(IReadOnlyList<ChannelInfo> channels)
+    {
+        var byParent = channels
+            .GroupBy(channel => channel.ParentId)
+            .ToDictionary(group => group.Key, group => group.OrderBy(channel => channel.SortOrder).ToArray());
+
+        IReadOnlyList<ChannelHierarchyItem> AddChildren(uint parentId)
+        {
+            if (!byParent.TryGetValue(parentId, out ChannelInfo[]? children))
+                return [];
+
+            return children
+                .Select(channel => new ChannelHierarchyItem(channel, AddChildren(channel.Id)))
+                .ToArray();
+        }
+
+        return AddChildren(0);
+    }
+}