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/SearchUnits.tsx

@@ -15,7 +15,7 @@ import {
   type Measure,
   type UnitInfo,
 } from '@/lib/units/units';
-import { cn } from '@/lib/units/utils';
+import { cn } from '@/lib/utils';
 
 interface SearchResult {
   unitInfo: UnitInfo;