fix(chat-input): replace input on slash-command insert

Cherry-pick from ChatTwo upstream ee7768ac (Infiziert90, 2026-05-16):
when args.AddIfNotPresent or args.Input starts with '/', replace the
chat input instead of appending. Fixes the Friend-List "/tell" path
where existing text like "test" would otherwise concatenate to
"test/tell user@world" before the receiver and channel resolve.

Variable drift versus upstream: HellionChat uses local 'Chat' where
ChatTwo uses InputHandler.ChatInput; logic is 1:1.

HellionChat/Ui/ChatLogWindow.cs

@@ -192,11 +192,28 @@ public sealed class ChatLogWindow : Window
             return;
         }
 
+        // ---------------------------------------------------------------
+        // Cherry-picked from ChatTwo upstream ee7768ac (Infiziert90, 2026-05-16)
+        // - Replace the chat input when args.AddIfNotPresent / args.Input starts
+        //   with a slash. Vanilla actions like the Friend List "/tell" entry and
+        //   other plugins push slash commands through these args; appending them
+        //   to existing text would produce inputs like "test/tell user@world".
+        // ---------------------------------------------------------------
         if (args.AddIfNotPresent != null && !Chat.Contains(args.AddIfNotPresent))
-            Chat += args.AddIfNotPresent;
+        {
+            if (args.AddIfNotPresent.StartsWith('/'))
+                Chat = args.AddIfNotPresent;
+            else
+                Chat += args.AddIfNotPresent;
+        }
 
         if (args.Input != null)
-            Chat += args.Input;
+        {
+            if (args.Input.StartsWith('/'))
+                Chat = args.Input;
+            else
+                Chat += args.Input;
+        }
 
         var (info, reason, target) = (args.ChannelSwitchInfo, args.TellReason, args.TellTarget);