improve chat input

[zeedann]

This pull request makes several improvements to the chat input area in the dashboard, focusing on visual consistency, accessibility, and user experience. The main changes include globalizing custom scrollbar styling, enhancing focus states for inputs, and redesigning the chat input composite for better usability and appearance.

Visual and UX improvements for chat input:

• Redesigned the chat input area in index.html to use a new .chat-input-wrap container, making the layout more visually appealing and consistent. The send button is now styled as a circular icon with a send arrow SVG, improving clarity and accessibility.
• Added new CSS for .chat-input-wrap and related elements to provide a cohesive background, border, and focus state, as well as improved placeholder styling for the textarea and focus-visible outlines for the send button.

Global styling enhancements:

• Updated custom scrollbar styles to apply globally across the dashboard (not just to .custom-scrollbar elements), ensuring consistent appearance in both WebKit and Firefox browsers. Increased scrollbar width and improved thumb styling.

Input focus accessibility:

• Improved input focus states by adding .dash-input:focus-within styles, ensuring that parent containers highlight when any child input is focused.