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

app/page.tsx

@@ -1,3 +1,6 @@
+'use client';
+
+import { useEffect } from 'react';
 import AnimatedBackground from '@/components/AnimatedBackground';
 import Hero from '@/components/Hero';
 import Stats from '@/components/Stats';
@@ -6,8 +9,14 @@ import Footer from '@/components/Footer';
 import BackToTop from '@/components/BackToTop';
 
 export default function Home() {
+  useEffect(() => {
+    // Force dark mode on html element for the landing page
+    document.documentElement.classList.remove('light');
+    document.documentElement.classList.add('dark');
+  }, []);
+
   return (
-    <main className="relative min-h-screen">
+    <main className="relative min-h-screen dark text-foreground">
       <AnimatedBackground />
       <BackToTop />
       <Hero />