feat: create header component with navigation

[nullcoder]

Create the main header component for GhostPaste with navigation and branding.

Requirements

Branding & Logo

• GhostPaste logo/text on the left (ghost emoji 👻 + "GhostPaste" text)
• Logo links to home page (/)
• Use consistent font and sizing from design system

Navigation Items

• "Create" - Links to /create (or just / if that's the create page)
• "About" - Links to /about or opens modal with project info
• "GitHub" - External link to https://github.com/nullcoder/ghostpaste

Theme Toggle

• Theme toggle button on the right side
• Use existing ThemeToggle component from feat: implement Web Crypto API encryption/decryption utilities #36
• Persist theme preference

Mobile Responsiveness

• Hamburger menu for screens < 768px
• Slide-out navigation drawer
• Close on outside click or escape key
• Smooth animations

Sticky Behavior

• Header becomes sticky on scroll
• Optional: Add subtle shadow when scrolled
• Ensure z-index properly layers over content

Accessibility

• Proper ARIA labels for navigation
• Skip to main content link
• Keyboard navigation support
• Focus management for mobile menu

Implementation Notes

// Use shadcn/ui components
import { NavigationMenu } from "@/components/ui/navigation-menu"
import { Sheet } from "@/components/ui/sheet" // for mobile drawer
import { ThemeToggle } from "@/components/theme-toggle"

Acceptance Criteria

• Header displays GhostPaste branding with ghost emoji
• Navigation links work correctly
• Theme toggle integrated and functional
• Mobile hamburger menu opens/closes smoothly
• Sticky header on scroll with proper z-index
• Keyboard navigation fully supported
• ARIA labels and accessibility features implemented
• Skip to main content link included
• Tested on multiple screen sizes

Technical Notes

• Use Next.js Link component for client-side navigation
• Implement with shadcn/ui NavigationMenu for desktop
• Use Sheet component for mobile drawer
• Support both light and dark themes
• Consider using CSS sticky positioning with fallback
• Test on actual mobile devices, not just browser DevTools