Widget: Glassmorphism, starter prompts, and dark mode fix

Starter prompts, glassmorphism UI, and a dark mode isolation fix for the widget.

• Configurable starter prompts at the top of new conversations
• Customers tap a prompt to populate the message field
• Glassmorphism effects: frosted glass backgrounds with subtle transparency and blur
• Browser tab title updates with unread count when the widget is in the background
• Fixed widget inheriting CSS from the parent page in dark mode
• Widget styles fully isolated from the host page