Update loading and error pages to new design system

[yusuftomilola]

Description:
Update the global loading.tsx, error.tsx, and newsletter/confirm/loading.tsx pages to use the new design system colors. Replace blue spinner borders and button colors with gray equivalents.

Acceptance Criteria:

• loading.tsx: spinner border uses border-gray-900
• error.tsx: retry button uses bg-gray-900 / hover:bg-gray-800
• newsletter/confirm/loading.tsx: background uses bg-[#faf9f7]

File(s): frontend/app/loading.tsx, frontend/app/error.tsx, frontend/app/newsletter/confirm/loading.tsx