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/providers/ThemeProvider.tsx

@@ -0,0 +1,77 @@
+'use client';
+
+import { createContext, useContext, useEffect, useState } from 'react';
+
+type Theme = 'dark' | 'light' | 'system';
+
+interface ThemeProviderProps {
+  children: React.ReactNode;
+  defaultTheme?: Theme;
+  storageKey?: string;
+}
+
+interface ThemeProviderState {
+  theme: Theme;
+  setTheme: (theme: Theme) => void;
+}
+
+const ThemeProviderContext = createContext<ThemeProviderState | undefined>(
+  undefined
+);
+
+export function ThemeProvider({
+  children,
+  defaultTheme = 'system',
+  storageKey = 'units-ui-theme',
+}: ThemeProviderProps) {
+  const [theme, setTheme] = useState<Theme>(defaultTheme);
+
+  useEffect(() => {
+    // Load theme from localStorage
+    const stored = localStorage.getItem(storageKey) as Theme | null;
+    if (stored) {
+      setTheme(stored);
+    }
+  }, [storageKey]);
+
+  useEffect(() => {
+    const root = window.document.documentElement;
+
+    root.classList.remove('light', 'dark');
+
+    if (theme === 'system') {
+      const systemTheme = window.matchMedia('(prefers-color-scheme: dark)')
+        .matches
+        ? 'dark'
+        : 'light';
+
+      root.classList.add(systemTheme);
+      return;
+    }
+
+    root.classList.add(theme);
+  }, [theme]);
+
+  const value = {
+    theme,
+    setTheme: (theme: Theme) => {
+      localStorage.setItem(storageKey, theme);
+      setTheme(theme);
+    },
+  };
+
+  return (
+    <ThemeProviderContext.Provider value={value}>
+      {children}
+    </ThemeProviderContext.Provider>
+  );
+}
+
+export const useTheme = () => {
+  const context = useContext(ThemeProviderContext);
+
+  if (context === undefined)
+    throw new Error('useTheme must be used within a ThemeProvider');
+
+  return context;
+};