Auto-send personalized follow-up emails after interviews

• New participant-email automation sends AI-personalized follow-ups after conversation completion, with a preview dialog and reply-to support.
• Welcome screen now accepts a typed first response when voice is disabled, auto-submitting once the conversation starts.
• Embed SDK gains brand color customization and light/dark/system theme overrides, with branded loaders.

• Thinking indicator now stays visible for any assistant message with reasoning, while animated phrases play only during streaming.
• Email preview is more responsive on mobile, with default-template tooltips and cleaner setup.
• Automations keep running gracefully when individual steps fail or no conversations exist.

Fixed style bleed between light and dark modes in embed theming and corrected brand color formats.