fix(webui): Make the Emotion Control Weight slider visible again

- The emotion weight is always applied in every mode except "Same as voice reference", so we must make the slider visible so that users can control the value. Otherwise it would silently apply the last-set value without the user knowing, which is very confusing.

- Furthermore, having the slider even on the Emotion Vectors page is *very* useful, because it allows users to rapidly change the total strength of the current emotion vectors without having to manually/carefully move every individual emotion slider.

webui.py

@@ -311,7 +311,7 @@ with gr.Blocks(title="IndexTTS Demo") as demo:
                     gr.update(visible=True),
                     gr.update(visible=True),
                     gr.update(visible=False),
-                    gr.update(visible=False)
+                    gr.update(visible=True)
                     )
         elif emo_control_method == 3:  # emotion text description
             return (gr.update(visible=False),