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/message/virtualized-message-thread.tsx

@@ -88,7 +88,7 @@ export function VirtualizedMessageThread<T>({
               key={getItemKey(item, index)}
               style={itemStyle(index, items.length)}
             >
-              <div className={cn("mx-auto max-w-3xl px-2", className)}>
+              <div className={cn("mx-auto max-w-3xl px-4", className)}>
                 {renderItem(item, index)}
               </div>
             </div>