fix(security): eliminate proxy/SSRF vulnerability and add comprehensi…

[SajanGhimire1]

…ve security headers

The previous next.config.ts contained unsafe rewrite rules that created an unauthenticated proxy/SSRF vulnerability:

// REMOVED - Security risk
rewrites: async function rewrites() {
  return [
    {
      source: '/api/:path*',
      destination: 'https://opensource.microsoft.com/api/:path*',
    },
    {
      source: '/avatars/:path*',
      destination: 'https://opensource.microsoft.com/avatars/:path*',
    },
  ]
}