fix(ui): unify scrollbar styles across scrollable containers

Add .scrollbar-thin and .scrollbar-thin-edge utility classes in
globals.css and apply them to sidebar, file tree, git changes,
git log, session files, diff preview, and message thread panels.
Replace scattered inline webkit-scrollbar overrides with the
shared classes for consistent appearance and gutter behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/files/file-workspace-panel.tsx

@@ -693,7 +693,7 @@ function DiffFileList({
           </p>
         )}
       </div>
-      <div className="flex-1 min-h-0 overflow-y-auto">
+      <div className="flex-1 min-h-0 overflow-y-auto scrollbar-thin">
         <div className="py-1">
           {diffOutline.files.map((file) => (
             <ContextMenu key={file.key}>