feat(frontend): replace native scrollbar styling with OverlayScrollbars

Adopt OverlayScrollbars for cross-platform consistent overlay scrollbars
with auto-hide on pointer leave, hover grow effect, and click-to-scroll.

- Add overlayscrollbars + overlayscrollbars-react dependencies
- Rewrite ScrollArea component from Radix to OverlayScrollbars wrapper
- Define custom theme `os-theme-codeg` in globals.css (6px → 8px on hover)
- Initialize body-level overlay scrollbar via OverlayScrollbarsInit
- Migrate all scrollbar-thin / scrollbar-thin-edge usages to ScrollArea
- Keep native .scrollbar-thin fallback for virtua scroll containers

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/layout/aux-panel-git-changes-tab.tsx

@@ -27,6 +27,7 @@ import {
   FileTreeFolder,
 } from "@/components/ai-elements/file-tree"
 import { Button } from "@/components/ui/button"
+import { ScrollArea } from "@/components/ui/scroll-area"
 import {
   ContextMenu,
   ContextMenuContent,
@@ -1287,7 +1288,7 @@ export function GitChangesTab() {
 
   return (
     <>
-      <div className="h-full min-h-0 overflow-y-auto scrollbar-thin-edge">
+      <ScrollArea className="h-full min-h-0" x="scroll">
         {trackedChanges.length === 0 && untrackedChanges.length === 0 ? (
           <div className="flex items-center justify-center h-full p-4">
             <p className="text-xs text-muted-foreground text-center">
@@ -1506,7 +1507,7 @@ export function GitChangesTab() {
             )}
           </div>
         )}
-      </div>
+      </ScrollArea>
 
       <Dialog
         open={Boolean(directoryGitActionType && directoryGitActionTarget)}