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/ui/ThemeToggle.tsx

@@ -0,0 +1,30 @@
+'use client';
+
+import { Moon, Sun } from 'lucide-react';
+import { useTheme } from '@/components/providers/ThemeProvider';
+import { Button } from '@/components/ui/button';
+
+export function ThemeToggle() {
+  const { theme, setTheme } = useTheme();
+
+  const toggleTheme = () => {
+    if (theme === 'dark') {
+      setTheme('light');
+    } else {
+      setTheme('dark');
+    }
+  };
+
+  return (
+    <Button
+      variant="outline"
+      size="icon"
+      onClick={toggleTheme}
+      className="relative"
+    >
+      <Sun className="h-5 w-5 rotate-0 scale-100 transition-all dark:-rotate-90 dark:scale-0" />
+      <Moon className="absolute h-5 w-5 rotate-90 scale-0 transition-all dark:rotate-0 dark:scale-100" />
+      <span className="sr-only">Toggle theme</span>
+    </Button>
+  );
+}