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/conversations/sidebar-conversation-list.tsx

@@ -482,7 +482,7 @@ export function SidebarConversationList({
             <div
               ref={scrollContainerRef}
               className={cn(
-                "flex-1 min-h-0 overflow-y-auto scrollbar-thin",
+                "flex-1 min-h-0 overflow-y-auto scrollbar-thin px-2",
                 "[overflow-anchor:none]"
               )}
             >