feat: add loading skeletons, error boundaries and not-found pages

[james-cross]

Summary

Add Next.js loading skeletons, error boundaries and not-found pages for all routes that fetch server-side data. This gives users immediate visual feedback during navigation and graceful fallbacks when things go wrong.

Changes

• Loading skeletons (loading.tsx) for 5 routes: homepage, location pages, organisation search, organisation detail, and resource pages. Each skeleton mirrors the actual page layout using Tailwind animate-pulse placeholder blocks.
• Error boundaries (error.tsx) for 3 routes: global app-level, location pages, and organisation detail. Each provides a "Try again" button via the reset() function and a contextual navigation link.
• Not-found pages (not-found.tsx) for 2 dynamic routes: location pages (/[slug]) and organisation detail (/find-help/organisation/[slug]). Each provides contextual messaging and navigation links.

Testing

• All 466 unit tests pass
• TypeScript compiles cleanly with no errors
• ESLint passes with no warnings
• Production build succeeds
• Loading states verified by navigating between routes in development