feat: implement navigation and dark/light theme system

Add complete navigation and theming infrastructure:

**Theme System:**
- ThemeProvider component with React Context
  - Supports light, dark, and system themes
  - Persists preference to localStorage
  - Auto-detects system theme preference
  - Listens for system theme changes
  - Smooth theme transitions

- ThemeToggle component
  - Sun/Moon icon toggle button
  - Accessible with ARIA labels
  - Shows current theme state
  - Keyboard accessible

**Navigation:**
- Navbar component with sticky header
  - Gradient logo with Palette icon
  - Desktop horizontal navigation
  - Mobile responsive menu
  - Active route highlighting
  - Backdrop blur effect
  - Links to all main sections:
    - Home
    - Playground
    - Palettes
    - Accessibility
    - Named Colors
    - Batch Operations

**Layout Updates:**
- Integrated Navbar into root layout
- Added ThemeProvider to Providers wrapper
- Proper HTML suppressHydrationWarning for theme
- Container-based responsive layout

**Features:**
- Theme persists across page reloads
- System theme preference detection
- Active navigation state
- Smooth hover transitions
- Mobile-first responsive design
- Accessible navigation with proper semantics

**Styling:**
- Gradient text logo (pink → purple → blue)
- Sticky top navbar with backdrop blur
- Border bottom for visual separation
- Consistent spacing and padding
- Mobile menu slides in smoothly

Build successful! Navigation and theming complete.

Next: Palette generation pages and additional features.

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

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

components/layout/ThemeToggle.tsx

@@ -0,0 +1,28 @@
+'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, resolvedTheme } = useTheme();
+
+  const toggleTheme = () => {
+    setTheme(resolvedTheme === 'dark' ? 'light' : 'dark');
+  };
+
+  return (
+    <Button
+      variant="ghost"
+      size="icon"
+      onClick={toggleTheme}
+      aria-label={`Switch to ${resolvedTheme === 'dark' ? 'light' : 'dark'} mode`}
+    >
+      {resolvedTheme === 'dark' ? (
+        <Sun className="h-5 w-5" />
+      ) : (
+        <Moon className="h-5 w-5" />
+      )}
+    </Button>
+  );
+}