feat: implement Phase 3 - Advanced UX features and interactivity

Add comprehensive UX enhancements with innovative features:

🔍 Fuzzy Search Component (SearchUnits.tsx):
- Powered by Fuse.js for intelligent fuzzy matching
- Searches across unit abbreviations, names, and categories
- Real-time dropdown with results
- Keyboard shortcut: Press "/" to focus search
- Press Escape to close
- Click outside to dismiss
- Shows measure category with color dot
- Top 10 results displayed
- Smart weighting: abbr (2x), singular/plural (1.5x), measure (1x)

💾 Conversion History (ConversionHistory.tsx):
- LocalStorage persistence (max 50 entries)
- Auto-saves conversions as user types
- Collapsible history panel
- Click to restore previous conversion
- Clear all with confirmation
- Shows relative time (just now, 5m ago, etc.)
- Live updates across tabs with storage events
- Custom event dispatch for same-window updates

🌓 Dark Mode Support:
- ThemeProvider with light/dark/system modes
- Persistent theme preference in localStorage
- Smooth theme transitions
- ThemeToggle component with animated sun/moon icons
- Gradient text adapts to theme
- System preference detection

⭐ Favorites & Copy Features:
- Star button to favorite units (localStorage)
- Copy to clipboard with visual feedback
- Hover to reveal action buttons
- Check icon confirmation for 2 seconds
- Yellow star fill for favorited units

⌨️ Keyboard Shortcuts:
- "/" - Focus search input
- "Escape" - Close search, blur inputs
- More shortcuts ready to add (Tab, Ctrl+K, etc.)

📦 LocalStorage Utilities (lib/storage.ts):
- saveToHistory() - Add conversion record
- getHistory() - Retrieve history
- clearHistory() - Clear all history
- getFavorites() / addToFavorites() / removeFromFavorites()
- toggleFavorite() - Toggle favorite status
- Type-safe ConversionRecord interface
- Automatic error handling

🎨 Enhanced MainConverter:
- Integrated search at top
- Conversion history at bottom
- Copy & favorite buttons on each result card
- Hover effects with opacity transitions
- Auto-save to history on conversion
- Click history item to restore conversion
- Visual feedback for all interactions

📱 Updated Layout & Page:
- ThemeProvider wraps entire app
- suppressHydrationWarning for SSR
- Top navigation bar with theme toggle
- Keyboard hint for search
- Dark mode gradient text variants

Dependencies Added:
- fuse.js 7.1.0 - Fuzzy search engine
- lucide-react 0.553.0 - Icon library (Search, Copy, Star, Check, etc.)

Features Now Working:
✅ Intelligent fuzzy search across 187 units
✅ Conversion history with persistence
✅ Dark mode with system detection
✅ Copy any result to clipboard
✅ Favorite units for quick access
✅ Keyboard shortcuts (/, Esc)
✅ Smooth animations and transitions
✅ Mobile-responsive design

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/converter/MainConverter.tsx

@@ -1,29 +1,41 @@
 'use client';
 
-import { useState, useEffect } from 'react';
+import { useState, useEffect, useCallback } from 'react';
+import { Copy, Star, Check } from 'lucide-react';
 import { Card, CardContent, CardHeader, CardTitle } from '@/components/ui/card';
 import { Input } from '@/components/ui/input';
 import { Button } from '@/components/ui/button';
+import SearchUnits from './SearchUnits';
+import ConversionHistory from './ConversionHistory';
 import {
   getAllMeasures,
   getUnitsForMeasure,
   convertToAll,
+  convertUnit,
   formatMeasureName,
   getCategoryColor,
   type Measure,
   type ConversionResult,
 } from '@/lib/units';
-import { parseNumberInput, formatNumber } from '@/lib/utils';
+import { parseNumberInput, formatNumber, cn } from '@/lib/utils';
+import { saveToHistory, getFavorites, toggleFavorite } from '@/lib/storage';
 
 export default function MainConverter() {
   const [selectedMeasure, setSelectedMeasure] = useState<Measure>('length');
   const [selectedUnit, setSelectedUnit] = useState<string>('m');
   const [inputValue, setInputValue] = useState<string>('1');
   const [conversions, setConversions] = useState<ConversionResult[]>([]);
+  const [favorites, setFavorites] = useState<string[]>([]);
+  const [copiedUnit, setCopiedUnit] = useState<string | null>(null);
 
   const measures = getAllMeasures();
   const units = getUnitsForMeasure(selectedMeasure);
 
+  // Load favorites
+  useEffect(() => {
+    setFavorites(getFavorites());
+  }, []);
+
   // Update conversions when input changes
   useEffect(() => {
     const numValue = parseNumberInput(inputValue);
@@ -43,8 +55,61 @@ export default function MainConverter() {
     }
   }, [selectedMeasure]);
 
+  // Copy to clipboard
+  const copyToClipboard = useCallback(async (value: number, unit: string) => {
+    try {
+      await navigator.clipboard.writeText(`${formatNumber(value)} ${unit}`);
+      setCopiedUnit(unit);
+      setTimeout(() => setCopiedUnit(null), 2000);
+    } catch (error) {
+      console.error('Failed to copy:', error);
+    }
+  }, []);
+
+  // Toggle favorite
+  const handleToggleFavorite = useCallback((unit: string) => {
+    const isFavorite = toggleFavorite(unit);
+    setFavorites(getFavorites());
+  }, []);
+
+  // Save to history when conversion happens
+  useEffect(() => {
+    const numValue = parseNumberInput(inputValue);
+    if (numValue !== null && selectedUnit && conversions.length > 0) {
+      // Save first conversion to history
+      const firstConversion = conversions.find(c => c.unit !== selectedUnit);
+      if (firstConversion) {
+        saveToHistory({
+          from: { value: numValue, unit: selectedUnit },
+          to: { value: firstConversion.value, unit: firstConversion.unit },
+          measure: selectedMeasure,
+        });
+        // Dispatch custom event for same-window updates
+        window.dispatchEvent(new Event('historyUpdated'));
+      }
+    }
+  }, [inputValue, selectedUnit, conversions, selectedMeasure]);
+
+  // Handle search selection
+  const handleSearchSelect = useCallback((unit: string, measure: Measure) => {
+    setSelectedMeasure(measure);
+    setSelectedUnit(unit);
+  }, []);
+
+  // Handle history selection
+  const handleHistorySelect = useCallback((record: any) => {
+    setInputValue(record.from.value.toString());
+    setSelectedMeasure(record.measure);
+    setSelectedUnit(record.from.unit);
+  }, []);
+
   return (
     <div className="w-full max-w-6xl mx-auto space-y-6">
+      {/* Search */}
+      <div className="flex justify-center">
+        <SearchUnits onSelectUnit={handleSearchSelect} />
+      </div>
+
       {/* Category Selection */}
       <Card>
         <CardHeader>
@@ -116,29 +181,66 @@ export default function MainConverter() {
         </CardHeader>
         <CardContent>
           <div className="grid grid-cols-1 md:grid-cols-2 lg:grid-cols-3 gap-4">
-            {conversions.map((conversion) => (
-              <div
-                key={conversion.unit}
-                className="p-4 rounded-lg border bg-card hover:bg-accent/50 transition-colors"
-                style={{
-                  borderLeftWidth: '4px',
-                  borderLeftColor: `var(--color-${getCategoryColor(selectedMeasure)})`,
-                }}
-              >
-                <div className="text-sm text-muted-foreground mb-1">
-                  {conversion.unitInfo.plural}
+            {conversions.map((conversion) => {
+              const isFavorite = favorites.includes(conversion.unit);
+              const isCopied = copiedUnit === conversion.unit;
+
+              return (
+                <div
+                  key={conversion.unit}
+                  className="group relative p-4 rounded-lg border bg-card hover:bg-accent/50 transition-colors"
+                  style={{
+                    borderLeftWidth: '4px',
+                    borderLeftColor: `var(--color-${getCategoryColor(selectedMeasure)})`,
+                  }}
+                >
+                  {/* Favorite & Copy buttons */}
+                  <div className="absolute top-2 right-2 flex gap-1 opacity-0 group-hover:opacity-100 transition-opacity">
+                    <Button
+                      variant="ghost"
+                      size="icon"
+                      className="h-8 w-8"
+                      onClick={() => handleToggleFavorite(conversion.unit)}
+                    >
+                      <Star
+                        className={cn(
+                          'h-4 w-4',
+                          isFavorite && 'fill-yellow-400 text-yellow-400'
+                        )}
+                      />
+                    </Button>
+                    <Button
+                      variant="ghost"
+                      size="icon"
+                      className="h-8 w-8"
+                      onClick={() => copyToClipboard(conversion.value, conversion.unit)}
+                    >
+                      {isCopied ? (
+                        <Check className="h-4 w-4 text-green-500" />
+                      ) : (
+                        <Copy className="h-4 w-4" />
+                      )}
+                    </Button>
+                  </div>
+
+                  <div className="text-sm text-muted-foreground mb-1">
+                    {conversion.unitInfo.plural}
+                  </div>
+                  <div className="text-2xl font-bold">
+                    {formatNumber(conversion.value)}
+                  </div>
+                  <div className="text-sm text-muted-foreground mt-1">
+                    {conversion.unit}
+                  </div>
                 </div>
-                <div className="text-2xl font-bold">
-                  {formatNumber(conversion.value)}
-                </div>
-                <div className="text-sm text-muted-foreground mt-1">
-                  {conversion.unit}
-                </div>
-              </div>
-            ))}
+              );
+            })}
           </div>
         </CardContent>
       </Card>
+
+      {/* Conversion History */}
+      <ConversionHistory onSelectConversion={handleHistorySelect} />
     </div>
   );
 }