refactor(DuoChat): consolidate web-only class usage in chat components

This MR consolidates the usage of the web-only class in chat components to improve CSS architecture and separation of concerns.

• Adds web-only class to root elements of both web_agentic_duo_chat and web_duo_chat components
• Refactors CSS from compound selector (.duo-chat-input.web-only) to descendant selector (.web-only .duo-chat-input)
• Removes redundant web-only classes from input elements since they now inherit web-specific styling from the root

This creates cleaner separation where the root-level web-only class controls all web-specific styling for the entire component, while maintaining identical functionality and allows for further web-only styles to plug in easily.

No visual changes.