fix: Enter key sends message when slash input matches no registered command

the Enter guard in the message composer checked !showCommands, but
showCommands is set to true whenever the input is a single word
starting with /. this blocked Enter from sending even when the typed
slash-word matched zero registered commands -- the dropdown was not
visible, yet the key was swallowed. this is a bug because it prevents
users from sending LLM-directed slash commands (e.g. /podcast, /admin)
that are not registered as Chainlit UI commands.

fix: only block Enter when the command dropdown is actually rendered,
i.e. when showCommands is true AND filteredCommands is non-empty. no
change in behavior when the slash-word matches a registered command --
the dropdown still captures Enter for selection as before.

Author: nzjrs

frontend/src/components/chat/MessageComposer/Input.tsx

@@ -154,13 +154,13 @@ const Input = forwardRef<InputMethods, Props>(
         }
       }
 
-      // Handle regular enter only if command menu is not showing
+      // Handle regular enter only if command dropdown is actually visible
       if (
         e.key === 'Enter' &&
         !e.shiftKey &&
         onEnter &&
         !isComposing &&
-        !showCommands
+        !(showCommands && filteredCommands.length > 0)
       ) {
         e.preventDefault();
         onEnter();