💄 style(ui): hide scrollbars across chat interface and request editor

Hide y-axis scrollbars to provide a cleaner UI experience while maintaining
scroll functionality through mouse wheel and keyboard navigation.

Changes include:
- Hide scrollbars in CustomRequestEditor TextArea component
- Hide scrollbars in chat container and all related chat components
- Hide scrollbars in thinking content areas
- Add cross-browser compatibility for scrollbar hiding
- Maintain scroll functionality while improving visual aesthetics

Components affected:
- CustomRequestEditor.js: Added custom-request-textarea class
- index.css: Updated scrollbar styles for chat, thinking, and editor areas

The interface now provides a more streamlined appearance consistent with
modern UI design patterns while preserving all interactive capabilities.

web/src/pages/Playground/index.js

@@ -363,7 +363,7 @@ const Playground = () => {
               flexShrink: 0,
               minWidth: styleState.isMobile ? '100%' : 320,
               maxWidth: styleState.isMobile ? '100%' : 320,
-              height: styleState.isMobile ? 'auto' : 'calc(100vh - 100px)',
+              height: styleState.isMobile ? 'auto' : 'calc(100vh - 106px)',
               overflow: 'auto',
               position: styleState.isMobile ? 'fixed' : 'relative',
               zIndex: styleState.isMobile ? 1000 : 1,
@@ -400,7 +400,7 @@ const Playground = () => {
         )}
 
         <Layout.Content className="relative flex-1 overflow-hidden">
-          <div className="sm:px-4 overflow-hidden flex flex-col lg:flex-row gap-2 sm:gap-4 h-[calc(100vh-100px)]">
+          <div className="sm:px-4 overflow-hidden flex flex-col lg:flex-row gap-2 sm:gap-4 h-[calc(100vh-106px)]">
             <div className="flex-1 flex flex-col">
               <ChatArea
                 chatRef={chatRef}