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/styles/theme.ts

@@ -193,7 +193,7 @@ const darkSemanticColors = {
   foregroundMuted: "#A1A5A4",
 
   // Controls
-  scrollbarHandle: "#71717a", // zinc-500
+  scrollbarHandle: "#717574", // zinc-500 w/ teal tint
 
   // Borders
   border: "#252B2A",