fix: themed scrollbar on message input and reusable scrollbar hooks

Add useWebElementScrollbar (for DOM elements) and useWebScrollViewScrollbar
(for RN ScrollView/FlatList) hooks that return renderable overlays, replacing
manual WebDesktopScrollbarOverlay wiring across all consumers. Apply the
themed scrollbar to the message input textarea. Tint the dark-mode scrollbar
handle to match the teal-tinted dark theme.

Closes #174

packages/app/src/components/web-desktop-scrollbar.tsx

@@ -37,7 +37,7 @@ function clamp(value: number, min: number, max: number): number {
   return Math.min(max, Math.max(min, value));
 }
 
-type ScrollbarMetrics = {
+export type ScrollbarMetrics = {
   offset: number;
   viewportSize: number;
   contentSize: number;