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>

pnpm-lock.yaml

@@ -107,6 +107,12 @@ importers:
       next-themes:
         specifier: ^0.4.6
         version: 0.4.6(react-dom@19.2.4(react@19.2.4))(react@19.2.4)
+      overlayscrollbars:
+        specifier: ^2.15.1
+        version: 2.15.1
+      overlayscrollbars-react:
+        specifier: ^0.5.6
+        version: 0.5.6(overlayscrollbars@2.15.1)(react@19.2.4)
       postcss:
         specifier: ^8.5.6
         version: 8.5.6
@@ -5227,6 +5233,15 @@ packages:
   outvariant@1.4.3:
     resolution: {integrity: sha512-+Sl2UErvtsoajRDKCE5/dBz4DIvHXQQnAxtQTF04OJxY0+DyZXSo5P5Bb7XYWOh81syohlYL24hbDwxedPUJCA==}
 
+  overlayscrollbars-react@0.5.6:
+    resolution: {integrity: sha512-E5To04bL5brn9GVCZ36SnfGanxa2I2MDkWoa4Cjo5wol7l+diAgi4DBc983V7l2nOk/OLJ6Feg4kySspQEGDBw==}
+    peerDependencies:
+      overlayscrollbars: ^2.0.0
+      react: '>=16.8.0'
+
+  overlayscrollbars@2.15.1:
+    resolution: {integrity: sha512-glX26JwjL+Tkzv0JNOWdW4VozP5dGXO+Wx8+TPrdTEJTSYT/8eJS8yXM+fewjU0nFq/JeCa+X+BqABNjC4YZSA==}
+
   own-keys@1.0.1:
     resolution: {integrity: sha512-qFOyK5PjiWZd+QQIh+1jhdb9LpxTF0qs7Pm8o5QHYZ0M3vKqSqzsZaEB6oWlxZ+q2sJBMI/Ktgd2N5ZwQoRHfg==}
     engines: {node: '>= 0.4'}
@@ -12402,6 +12417,13 @@ snapshots:
 
   outvariant@1.4.3: {}
 
+  overlayscrollbars-react@0.5.6(overlayscrollbars@2.15.1)(react@19.2.4):
+    dependencies:
+      overlayscrollbars: 2.15.1
+      react: 19.2.4
+
+  overlayscrollbars@2.15.1: {}
+
   own-keys@1.0.1:
     dependencies:
       get-intrinsic: 1.3.0