Turn off the post-rating feedback follow-up link, plus a dark mode fix for voice alerts

• You can now turn off the "Tell us why?" follow-up link that appears after participants rate their conversation experience. The rating options always stay visible, so you keep the quick pulse without sending participants off to a separate feedback page.
• The control for changing a conversation's background scene is easier to spot, with a clearer icon and a matching "Scene" label across desktop and mobile.

Voice conversations run on a dark screen, but popup alerts like "microphone blocked" were still showing in light mode, flashing up as bright boxes that looked broken. They now match the dark theme, so the screen stays consistent.