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>

package.json

@@ -44,6 +44,8 @@
     "next": "^16",
     "next-intl": "^4.8.3",
     "next-themes": "^0.4.6",
+    "overlayscrollbars": "^2.15.1",
+    "overlayscrollbars-react": "^0.5.6",
     "postcss": "^8.5.6",
     "radix-ui": "^1.4.3",
     "react": "^19.1.0",