perf: add content-visibility: auto to long list items

Apply content-visibility: auto to off-screen list and sidebar items
to skip browser layout and painting until they scroll into view.

Changes:
- globals.css: add .list-item-lazy (48px) and .sidebar-item-lazy (40px) utilities
- DocumentsTableShell: apply list-item-lazy to desktop <tr> rows and mobile card divs
- AllPrivateChatsSidebar: apply sidebar-item-lazy to thread items
- AllSharedChatsSidebar: apply sidebar-item-lazy to thread items
- InboxSidebar: apply content-visibility inline (80px intrinsic height) to inbox items

Implements Vercel React Best Practices Rule: rendering-content-visibility (6.2)

surfsense_web/components/layout/ui/sidebar/AllPrivateChatsSidebar.tsx

@@ -349,7 +349,7 @@ export function AllPrivateChatsSidebarContent({
 								<div
 									key={thread.id}
 									className={cn(
-										"group flex items-center gap-2 rounded-md px-2 py-1.5 text-sm",
+										"sidebar-item-lazy group flex items-center gap-2 rounded-md px-2 py-1.5 text-sm",
 										"hover:bg-accent hover:text-accent-foreground",
 										"transition-colors cursor-pointer",
 										isActive && "bg-accent text-accent-foreground",