feat: add custom 404 and 500 error pages closes #68

[AYo101o]

🛠️ Custom 404 & 500 Error Pages

Closes #68

What this PR does

Adds custom not-found.tsx (404) and error.tsx (500) pages styled consistently with Nevo's dark theme, replacing Next.js default error screens.

404 – Page Not Found

• Large styled error code with "Page Not Found" heading
• Friendly description guiding the user
• Back to Home button with arrow icon
• Quick links to Homepage and Contact Support

500 – Server Error

• Clear "Server Error" heading with support message
• Try Again button that calls Next.js reset() to retry rendering
• Back to Home button as fallback
• Dev-only error message block showing error.message for easier debugging
• Contact Support link at the bottom

Design

• Consistent with Nevo's color palette (#0a1428 bg, #1fe4ff accent, #10b981 green)
• Fully responsive across mobile, tablet, and desktop
• Decorative dot elements for visual consistency

Notes

• 500 page couldn't be live-tested as the backend isn't set up yet, but the component handles the error and reset props as per Next.js App Router spec

[AYo101o]