feat: add route-level code splitting with React.lazy and Suspense (Closes #469)

[waterWang]

Summary

Converts all 20 page components in App.tsx from eager imports to React.lazy() dynamic imports, enabling route-level code splitting. Each page chunk loads on demand, reducing the initial bundle size.

Changes

Code Splitting

• All 20 page imports replaced with React.lazy(() => import('./pages/...'))
• Layout/utility components (EmployerLayout, ErrorBoundary, ErrorFallback) remain as regular imports since they are not route pages
• Suspense wraps the entire <Routes> block with a centered loading spinner (Loader2 from lucide-react)

Loading Fallback

• PageLoader component shows a centered spinner with "Loading…" text
• Uses Loader2 with animate-spin from the existing lucide-react dependency

Error Handling

• Each route already has its own ErrorBoundary wrapper — chunk load failures are caught by the existing error boundary hierarchy
• The outer Suspense fallback only shows during initial chunk loading; runtime errors are handled by the per-route ErrorBoundary

Bundle Impact

• Vite automatically splits each lazy import into a separate chunk
• The initial HTML/JS payload no longer includes all 20 page components
• Verified with tsc --noEmit — zero type errors

Verification

• tsc --noEmit passes cleanly
• lucide-react is already a project dependency (Loader2 used in CrossAssetPayment.tsx)
• Pattern matches Vite's recommended code-splitting approach

Closes #469