Add global dark mode footer

[Adeolu01]

Summary

Implements a reusable GlobalFooter component matching the Dark Mode design system, following the project's Component -> Hook -> Service architecture.

• Three primary link sections: Infrastructure, Resources, Connect
• Email subscription form with client-side validation and pending/success/error states
• Bottom legal section with Privacy Policy, Terms of Service, Cookie Policy links and copyright line
• On mobile, the three sections collapse into an accessible accordion (aria-expanded/aria-controls); on desktop they render as a grid
• Full dark mode support via Tailwind dark: classes

Architecture

• Service: services/footerService.ts - getFooterContent() and subscribeToNewsletter(email), axios-based, follows the existing faqService.ts pattern
• Hook: hooks/useFooter.ts - useFooter() (react-query) for content, useNewsletterSubscribe() (mutation) for the subscribe form
• Component: components/shared/GlobalFooter.tsx - consumes the hooks only, no direct service calls, no inline mock content driving the primary render path

Integration

• Wired GlobalFooter into the root layout (app/layout.tsx) so it renders once across the app
• Removed the now-redundant inline footer markup from app/page.tsx to avoid duplication

Testing

• npx tsc --noEmit: no errors in files touched by this change (pre-existing unrelated errors elsewhere in the repo are untouched)
• npx eslint on the changed files: 0 errors, 0 warnings
• Manually verified accordion behavior, subscribe form validation states, and dark mode styling

Notes

• Backend endpoints /api/footer and /api/footer/subscribe are assumed per the issue spec. Until they exist, the footer falls back to a static set of sections for rendering and the subscribe form surfaces its error state on submit.

closes #306

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