Widget: Glassmorphism, starter prompts, and dark mode fix

Visual polish and practical improvements to the widget experience.

What's new

• Starter prompts at the beginning of new conversations to help customers ask the right questions.
• Glassmorphism effects and polished hover states across widget interactions.
• The browser tab title now updates with unread message counts.
• Fixed an issue where the widget inherited the parent page's dark mode, so it now looks correct regardless of the host site's theme.