Improve error page design to match brand

[teslims2]

Description

The global error page is plain. Redesign it to match the StellarKraal brand with an illustration and better typography.

This is a frontend task in the Next.js 14 + Tailwind CSS codebase. All UI changes must respect the existing design token system, support both light and dark mode, and meet WCAG AA contrast requirements. New or modified components should be covered by React Testing Library unit tests.

Acceptance Criteria

• Error page uses the app's design tokens for colours and typography
• Includes a livestock-themed error illustration
• Clear heading and subheading text
• 'Go Home' and 'Try Again' action buttons
• Renders correctly in both light and dark mode
• Unit/integration tests added or updated to cover the change
• All existing CI checks pass with the change applied
• Component is keyboard-navigable and screen-reader friendly