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/MainConverter.tsx

@@ -8,7 +8,7 @@ import { Button } from '@/components/ui/Button';
 import SearchUnits from './SearchUnits';
 import ConversionHistory from './ConversionHistory';
 import VisualComparison from './VisualComparison';
-import CommandPalette from '@/components/ui/CommandPalette';
+import CommandPalette from '@/components/units/ui/CommandPalette';
 import {
   getAllMeasures,
   getUnitsForMeasure,
@@ -20,7 +20,7 @@ import {
   type Measure,
   type ConversionResult,
 } from '@/lib/units/units';
-import { parseNumberInput, formatNumber, cn } from '@/lib/units/utils';
+import { parseNumberInput, formatNumber, cn } from '@/lib/utils';
 import { saveToHistory, getFavorites, toggleFavorite } from '@/lib/units/storage';
 
 export default function MainConverter() {