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-session-files-tab.tsx

@@ -103,7 +103,7 @@ function SessionFilesContent({ conversationId }: { conversationId: number }) {
   }
 
   return (
-    <div className="space-y-3 p-3">
+    <div className="space-y-3 px-1 py-3">
       {groups.map((group, groupIndex) => {
         const groupKey = `${group.userTurnId}-${group.timestamp}-${groupIndex}`
         const isOpen = openGroups[groupKey] ?? false
@@ -290,7 +290,7 @@ export function SessionFilesTab() {
 
   return (
     <div className="flex flex-col h-full">
-      <div className="flex-1 min-h-0 overflow-y-auto">
+      <div className="flex-1 min-h-0 overflow-y-auto scrollbar-thin">
         <SessionFilesContent conversationId={conversationId} />
       </div>
     </div>