feat: implement Phase 4 - Command palette, visual comparison, and polish

Add final layer of advanced features for spectacular UX:

⌨️ Command Palette (CommandPalette.tsx):
- Trigger with Ctrl/Cmd+K keyboard shortcut
- Search and execute commands instantly
- Quick access to all 23 measurement categories
- Theme switching commands (light/dark/system)
- Keyboard navigation (↑/↓ arrows, Enter to select)
- Escape to close
- Color-coded category commands
- Backdrop blur overlay
- Animated scale-in entrance
- Footer with keyboard hints
- Smart filtering by keywords

📊 Visual Comparison (VisualComparison.tsx):
- Toggle between grid and chart view
- Horizontal bar chart showing magnitude differences
- Animated bars with 500ms transitions
- Auto-calculated percentages relative to max value
- Color-coded bars matching category colors
- Tabular number formatting
- Clean, minimal design

🔄 Unit Swap Functionality:
- Swap button between From/To units
- ArrowLeftRight icon
- Automatically converts value on swap
- Two-unit quick converter mode
- Large result display with color accent
- Responsive layout

📱 Footer Component (Footer.tsx):
- Three-column grid layout (About, Features, Links)
- GitHub repository link
- convert-units library attribution
- Keyboard shortcuts reference
- Feature highlights
- Made with ❤️ message
- Responsive design (stacks on mobile)
- Copyright notice with current year
- Smooth hover transitions

🎨 Enhanced MainConverter:
- From/To unit selector with swap button
- Quick result display between selectors
- Toggle between grid and chart views
- BarChart3 icon for view switcher
- Integrated command palette
- Better spacing and layout
- Target unit state management
- Auto-update target unit on measure change

✨ Enhanced Page Layout:
- Sticky header with backdrop blur
- Flexbox layout for footer at bottom
- Keyboard shortcuts hint (/ and Ctrl+K)
- Improved header spacing
- Better visual hierarchy

Features Now Complete:
✅ Command palette with Ctrl+K
✅ Visual comparison bar charts
✅ Unit swap functionality
✅ Professional footer
✅ From/To quick converter
✅ Chart/Grid view toggle
✅ Sticky navigation header
✅ Full keyboard navigation

The app is now feature-complete with:
- 23 measurement categories
- 187 individual units
- Real-time conversion
- Fuzzy search (/)
- Command palette (Ctrl+K)
- Dark mode
- Conversion history
- Favorites & copy
- Visual comparisons
- Unit swapping
- Complete footer

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

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

app/page.tsx

@@ -1,18 +1,19 @@
 import MainConverter from '@/components/converter/MainConverter';
 import { ThemeToggle } from '@/components/ui/ThemeToggle';
+import Footer from '@/components/ui/Footer';
 
 export default function Home() {
   return (
-    <div className="min-h-screen bg-background">
+    <div className="min-h-screen bg-background flex flex-col">
       {/* Header with theme toggle */}
-      <div className="border-b">
+      <div className="border-b sticky top-0 bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/60 z-40">
         <div className="container mx-auto px-4 py-4 flex justify-between items-center">
           <div className="text-xl font-bold">Units UI</div>
           <ThemeToggle />
         </div>
       </div>
 
-      <main className="container mx-auto px-4 py-8">
+      <main className="container mx-auto px-4 py-8 flex-1">
         <header className="text-center mb-12">
           <h1 className="text-5xl font-bold mb-4 bg-gradient-to-r from-blue-600 to-indigo-600 dark:from-blue-400 dark:to-indigo-400 bg-clip-text text-transparent">
             Unit Converter
@@ -20,13 +21,21 @@ export default function Home() {
           <p className="text-lg text-muted-foreground">
             Convert between 187 units across 23 measurement categories
           </p>
-          <p className="text-sm text-muted-foreground mt-2">
-            Press <kbd className="px-2 py-1 bg-muted rounded text-xs">/</kbd> to search units
-          </p>
+          <div className="flex items-center justify-center gap-4 mt-3 text-sm text-muted-foreground">
+            <span>Press <kbd className="px-2 py-1 bg-muted rounded text-xs">/</kbd> to search</span>
+            <span>•</span>
+            <span>
+              <kbd className="px-2 py-1 bg-muted rounded text-xs">Ctrl</kbd>
+              {' + '}
+              <kbd className="px-2 py-1 bg-muted rounded text-xs">K</kbd> for commands
+            </span>
+          </div>
         </header>
 
         <MainConverter />
       </main>
+
+      <Footer />
     </div>
   );
 }