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-file-tree-tab.tsx

@@ -2167,7 +2167,7 @@ export function FileTreeTab() {
     <div className="flex flex-col h-full">
       <ContextMenu>
         <ContextMenuTrigger asChild>
-          <div className="flex-1 min-h-0 overflow-auto pb-1 [scrollbar-gutter:stable] [&::-webkit-scrollbar]:h-1.5 [&::-webkit-scrollbar]:w-1.5 [&::-webkit-scrollbar-thumb]:rounded-full [&::-webkit-scrollbar-thumb]:bg-border">
+          <div className="flex-1 min-h-0 overflow-auto pb-1 scrollbar-thin-edge">
             <FileTree
               key={folder?.path ?? "file-tree-empty"}
               className="border-0 rounded-none bg-transparent w-max min-w-full"