feat(safety): refuse to load when Chat 2 is also active

When the user has both Hellion Chat and upstream Chat 2 installed and
loaded, the parallel chat-window replacement and Hook collisions can
crash FFXIV at frame boundaries. The new detector inspects
IDalamudPluginInterface.InstalledPlugins and throws an
InvalidOperationException with a localized message if Chat 2 is loaded,
which Dalamud surfaces cleanly instead of letting the load proceed
into a runtime crash.

Bilingual messages (EN/DE) follow the existing HellionStrings pattern.

ChatTwo/ChatTwoConflictDetector.cs

@@ -0,0 +1,27 @@
+using System.Linq;
+using ChatTwo.Resources;
+using Dalamud.Plugin;
+
+namespace ChatTwo;
+
+internal static class ChatTwoConflictDetector
+{
+    private const string UpstreamInternalName = "ChatTwo";
+
+    public static void ThrowIfChatTwoIsLoaded(IDalamudPluginInterface pluginInterface)
+    {
+        var conflict = pluginInterface.InstalledPlugins
+            .FirstOrDefault(p =>
+                p.InternalName == UpstreamInternalName &&
+                p.IsLoaded);
+
+        if (conflict is null)
+            return;
+
+        var message = HellionStrings.ChatTwoConflictTitle + "\n\n" +
+                      HellionStrings.ChatTwoConflictBody + "\n\n" +
+                      HellionStrings.ChatTwoConflictAction;
+
+        throw new System.InvalidOperationException(message);
+    }
+}