refactor(web): simplify sidebar visual design

- Remove vertical guide lines from collapsible sub-items (border-l)
- Move create button from list bottom to category header row as a hover-revealed + icon
- Remove active background highlight from category headers; only child entities show active state
- Remove unused CREATE_I18N_KEYS constant

web/src/components/ui/sidebar.tsx

@@ -651,7 +651,7 @@ function SidebarMenuSub({ className, ...props }: React.ComponentProps<'ul'>) {
       data-slot="sidebar-menu-sub"
       data-sidebar="menu-sub"
       className={cn(
-        'border-sidebar-border mx-3.5 flex min-w-0 translate-x-px flex-col gap-1 border-l px-2.5 py-0.5',
+        'mx-3.5 flex min-w-0 translate-x-px flex-col gap-1 px-2.5 py-0.5',
         'group-data-[collapsible=icon]:hidden',
         className,
       )}