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/units/converter/VisualComparison.tsx

@@ -2,7 +2,7 @@
 
 import { useMemo, useState, useRef, useCallback, useEffect } from 'react';
 import { type ConversionResult } from '@/lib/units/units';
-import { formatNumber, cn } from '@/lib/units/utils';
+import { formatNumber, cn } from '@/lib/utils';
 
 interface VisualComparisonProps {
   conversions: ConversionResult[];