Add Stationery and Stationery Dark themes with notebook chrome.

Manila folders, ruled chat, post-it nav, and folder tabs; dark variant uses Catppuccin Mocha colors with muted stickies and soft glows.

src/app/components/sidebar/Sidebar.tsx

@@ -4,5 +4,10 @@ import React from 'react';
 import * as css from './Sidebar.css';
 
 export const Sidebar = as<'div'>(({ as: AsSidebar = 'div', className, ...props }, ref) => (
-  <AsSidebar className={classNames(css.Sidebar, className)} {...props} ref={ref} />
+  <AsSidebar
+    className={classNames(css.Sidebar, className)}
+    data-sidebar=""
+    {...props}
+    ref={ref}
+  />
 ));