feat: initialize Next.js 16 project with Tailwind CSS 4 and Docker support

Phase 1 Implementation:
- Set up Next.js 16 with React 19, TypeScript 5, and Turbopack
- Configure Tailwind CSS 4 with OKLCH color system
- Implement dark/light theme support
- Create core UI components: Button, Card, Slider, Progress, Toast
- Add ThemeToggle component for theme switching
- Set up project directory structure for audio editor
- Create storage utilities for settings management
- Add Dockerfile with multi-stage build (Node + nginx)
- Configure nginx for static file serving with caching
- Add docker-compose.yml for easy deployment
- Configure static export mode for production

Tech Stack:
- Next.js 16 with Turbopack
- React 19
- TypeScript 5
- Tailwind CSS 4
- pnpm 9.0.0
- nginx 1.27 (for Docker deployment)

Build verified and working ✓

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

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

components/layout/ThemeToggle.tsx

@@ -0,0 +1,43 @@
+'use client';
+
+import * as React from 'react';
+import { Moon, Sun } from 'lucide-react';
+import { Button } from '@/components/ui/Button';
+
+export function ThemeToggle() {
+  const [theme, setTheme] = React.useState<'light' | 'dark'>('light');
+
+  React.useEffect(() => {
+    // Get initial theme
+    const isDark = document.documentElement.classList.contains('dark');
+    setTheme(isDark ? 'dark' : 'light');
+  }, []);
+
+  const toggleTheme = () => {
+    const newTheme = theme === 'light' ? 'dark' : 'light';
+    setTheme(newTheme);
+
+    if (newTheme === 'dark') {
+      document.documentElement.classList.add('dark');
+      localStorage.setItem('theme', 'dark');
+    } else {
+      document.documentElement.classList.remove('dark');
+      localStorage.setItem('theme', 'light');
+    }
+  };
+
+  return (
+    <Button
+      variant="ghost"
+      size="icon"
+      onClick={toggleTheme}
+      title={`Switch to ${theme === 'light' ? 'dark' : 'light'} mode`}
+    >
+      {theme === 'light' ? (
+        <Moon className="h-5 w-5" />
+      ) : (
+        <Sun className="h-5 w-5" />
+      )}
+    </Button>
+  );
+}