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/conversations/sidebar-conversation-list.tsx

@@ -482,11 +482,8 @@ export function SidebarConversationList({
             <div
               ref={scrollContainerRef}
               className={cn(
-                "flex-1 min-h-0 overflow-y-auto px-1.5",
-                "[overflow-anchor:none]",
-                "[&::-webkit-scrollbar]:w-1.5",
-                "[&::-webkit-scrollbar-thumb]:rounded-full",
-                "[&::-webkit-scrollbar-thumb]:bg-border"
+                "flex-1 min-h-0 overflow-y-auto scrollbar-thin",
+                "[overflow-anchor:none]"
               )}
             >
               <Virtualizer ref={virtualizerRef} itemSize={CARD_HEIGHT}>