fix(frontend): remove scrollbar-gutter and restore padding for scroll containers

Drop `scrollbar-gutter: stable both-edges` / `stable` from `.scrollbar-thin`
and `.scrollbar-thin-edge` so scrollbars overlay content without reserving
space, fixing layout inconsistencies between overlay and classic scrollbar
environments. Restore padding that was previously reduced to compensate for the
gutter: `px-1` to `px-3` in git-log-tab, `px-2` to `px-4` in
virtualized-message-thread, and add `px-2` to sidebar-conversation-list and
session-files-tab.

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

src/components/layout/aux-panel-session-files-tab.tsx

@@ -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 scrollbar-thin">
+      <div className="flex-1 min-h-0 overflow-y-auto scrollbar-thin px-2">
         <SessionFilesContent conversationId={conversationId} />
       </div>
     </div>