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/layout/aux-panel-git-changes-tab.tsx

@@ -1287,7 +1287,7 @@ export function GitChangesTab() {
 
   return (
     <>
-      <div className="h-full min-h-0 overflow-y-auto">
+      <div className="h-full min-h-0 overflow-y-auto scrollbar-thin-edge">
         {trackedChanges.length === 0 && untrackedChanges.length === 0 ? (
           <div className="flex items-center justify-center h-full p-4">
             <p className="text-xs text-muted-foreground text-center">