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/overlay-scrollbars-init.tsx

@@ -0,0 +1,25 @@
+"use client"
+
+import { useEffect } from "react"
+import "overlayscrollbars/overlayscrollbars.css"
+import { useOverlayScrollbars } from "overlayscrollbars-react"
+
+export function OverlayScrollbarsInit() {
+  const [init] = useOverlayScrollbars({
+    options: {
+      scrollbars: {
+        theme: "os-theme-codeg",
+        autoHide: "leave",
+        clickScroll: true,
+      },
+      overflow: { x: "hidden" },
+    },
+    defer: true,
+  })
+
+  useEffect(() => {
+    init(document.body)
+  }, [init])
+
+  return null
+}