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/file-pane.tsx

@@ -13,6 +13,7 @@ import { useIsCompactFormFactor } from "@/constants/layout";
 import { Fonts } from "@/constants/theme";
 import { useSessionStore, type ExplorerFile } from "@/stores/session-store";
 import { useWebScrollViewScrollbar } from "@/components/use-web-scrollbar";
+import { useWebScrollbarStyle } from "@/hooks/use-web-scrollbar-style";
 import {
   highlightCode,
   darkHighlightColors,
@@ -125,6 +126,7 @@ function FilePreviewBody({
   const isMarkdownFile = preview?.kind === "text" && isRenderedMarkdownFile(filePath);
 
   const previewScrollRef = useRef<RNScrollView>(null);
+  const webScrollbarStyle = useWebScrollbarStyle();
   const scrollbar = useWebScrollViewScrollbar(previewScrollRef, {
     enabled: showDesktopWebScrollbar,
   });
@@ -216,6 +218,7 @@ function FilePreviewBody({
               horizontal
               nestedScrollEnabled
               showsHorizontalScrollIndicator
+              style={webScrollbarStyle}
               contentContainerStyle={styles.previewCodeScrollContent}
             >
               {codeLines}