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

components/layout/AppShell.tsx

@@ -13,7 +13,7 @@ interface AppShellProps {
 export function AppShell({ children }: AppShellProps) {
   return (
     <SidebarProvider>
-      <div className="flex min-h-screen bg-background text-foreground relative">
+      <div className="flex h-screen overflow-hidden bg-background text-foreground relative">
         <AnimatedBackground />
         <AppSidebar />
         <div className="flex-1 flex flex-col min-w-0 relative z-10">