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/ConversionHistory.tsx

@@ -9,7 +9,7 @@ import {
   clearHistory,
   type ConversionRecord,
 } from '@/lib/units/storage';
-import { getRelativeTime, formatNumber } from '@/lib/units/utils';
+import { getRelativeTime, formatNumber } from '@/lib/utils';
 import { formatMeasureName } from '@/lib/units/units';
 
 interface ConversionHistoryProps {