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/hooks/use-web-scrollbar-style.d.ts

@@ -0,0 +1 @@
+export * from "./use-web-scrollbar-style.web";

packages/app/src/hooks/use-web-scrollbar-style.native.ts

@@ -0,0 +1,5 @@
+import type { ViewStyle } from "react-native";
+
+export function useWebScrollbarStyle(): ViewStyle | undefined {
+  return undefined;
+}

packages/app/src/hooks/use-web-scrollbar-style.web.ts

@@ -0,0 +1,21 @@
+import { useMemo } from "react";
+import type { ViewStyle } from "react-native";
+import { useUnistyles } from "react-native-unistyles";
+
+// CSS scrollbar properties are supported by React Native Web at runtime
+// but are not included in React Native's ViewStyle type definition.
+interface WebScrollbarStyle extends ViewStyle {
+  scrollbarColor: string;
+  scrollbarWidth: string;
+}
+
+export function useWebScrollbarStyle(): WebScrollbarStyle {
+  const { theme } = useUnistyles();
+  return useMemo(
+    (): WebScrollbarStyle => ({
+      scrollbarColor: `${theme.colors.scrollbarHandle} transparent`,
+      scrollbarWidth: "thin",
+    }),
+    [theme.colors.scrollbarHandle],
+  );
+}