Update chat-header and chat-input-text-field CSS to match the
Dart counterparts (chat_header.dart, chat_input_text_field.dart):
- Header: remove outer padding/background, use transparent base
- Guest banner: switch to --spacing-sm/md tokens
- Header row: add margin-left and symmetric padding
- Menu button: use 32px radius and #0d0b14 @ 0.7 background
- Input field: add white background, 32px radius, accent caret
color, and flex centering for vertical text alignment