style(app): theme native scrollbars across all web views (#399)

* style(app): theme native scrollbars in tool call detail views

Apply CSS scrollbar-color to all ScrollViews in ToolCallDetailsContent
and DiffViewer so the browser scrollbar matches the app's dark theme
instead of showing the default white native scrollbar.

* refactor: split useWebScrollbarStyle into .web.ts/.native.ts platform files

- Native shim returns undefined without calling useUnistyles
- Web variant uses properly typed WebScrollbarStyle interface instead of `as any`
- Add .d.ts for TypeScript module resolution

---------

Co-authored-by: Mohamed Boudra <boudra.moha@gmail.com>

packages/app/src/components/diff-scroll.web.tsx

@@ -1,4 +1,5 @@
 import { ScrollView, type LayoutChangeEvent, type StyleProp, type ViewStyle } from "react-native";
+import { useWebScrollbarStyle } from "@/hooks/use-web-scrollbar-style";
 
 interface DiffScrollProps {
   children: React.ReactNode;
@@ -14,12 +15,14 @@ export function DiffScroll({
   style,
   contentContainerStyle,
 }: DiffScrollProps) {
+  const webScrollbarStyle = useWebScrollbarStyle();
+
   return (
     <ScrollView
       horizontal
       nestedScrollEnabled
       showsHorizontalScrollIndicator
-      style={style}
+      style={[style, webScrollbarStyle]}
       contentContainerStyle={contentContainerStyle}
       onLayout={(e: LayoutChangeEvent) => onScrollViewWidthChange(e.nativeEvent.layout.width)}
     >