Don AI Chatbot 4.2.3

Mobile live-chat improvements:
- Compact sticky mobile action bar with icon buttons for Join, AI, Restart, and End.
- Visitor focus pill: Viewing chat, Browsing website, Tab hidden, or Left site.
- Visitor details become collapsible on mobile to keep the conversation first.
- Message area gets more usable vertical space on phone screens.
- Visitor presence ping now sends chat-open state, page visibility, current URL, and page title.
- Admin current page link updates live from visitor presence when available.
