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 {

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() {

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;

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[];