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-file-tree-tab.tsx

@@ -37,6 +37,7 @@ import {
   stopFileTreeWatch,
 } from "@/lib/api"
 import { emitAttachFileToSession } from "@/lib/session-attachment-events"
+import { ScrollArea } from "@/components/ui/scroll-area"
 import type {
   FileTreeChangedEvent,
   FileTreeNode,
@@ -2167,7 +2168,7 @@ export function FileTreeTab() {
     <div className="flex flex-col h-full">
       <ContextMenu>
         <ContextMenuTrigger asChild>
-          <div className="flex-1 min-h-0 overflow-auto pb-1 scrollbar-thin-edge">
+          <ScrollArea className="flex-1 min-h-0 pb-1" x="scroll">
             <FileTree
               key={folder?.path ?? "file-tree-empty"}
               className="border-0 rounded-none bg-transparent w-max min-w-full"
@@ -2319,7 +2320,7 @@ export function FileTreeTab() {
                 </ContextMenu>
               )}
             </FileTree>
-          </div>
+          </ScrollArea>
         </ContextMenuTrigger>
         <ContextMenuContent>
           <ContextMenuSub>