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-session-files-tab.tsx

@@ -18,6 +18,7 @@ import {
   CollapsibleContent,
   CollapsibleTrigger,
 } from "@/components/ui/collapsible"
+import { ScrollArea } from "@/components/ui/scroll-area"
 import { cn } from "@/lib/utils"
 
 function isRemovedFileDiff(diff: string | null): boolean {
@@ -290,9 +291,9 @@ export function SessionFilesTab() {
 
   return (
     <div className="flex flex-col h-full">
-      <div className="flex-1 min-h-0 overflow-y-auto scrollbar-thin px-2">
+      <ScrollArea className="flex-1 min-h-0 px-2">
         <SessionFilesContent conversationId={conversationId} />
-      </div>
+      </ScrollArea>
     </div>
   )
 }