refactor: consolidate utilities, clean up components, and improve theme persistence

- Consolidate common utilities (cn, format, time) into lib/utils
- Remove redundant utility files from pastel and units directories
- Clean up unused components (Separator, KeyboardShortcutsHelp)
- Relocate CommandPalette to components/units/ui/
- Force dark mode on landing page and improve theme persistence logic
- Add FOUC prevention script to RootLayout
- Fix sidebar height constraint in AppShell

lib/pastel/utils/cn.ts

@@ -1,6 +0,0 @@
-import { clsx, type ClassValue } from 'clsx';
-import { twMerge } from 'tailwind-merge';
-
-export function cn(...inputs: ClassValue[]) {
-  return twMerge(clsx(inputs));
-}