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

[waterWang]

Summary

Closes #469 — Add route-level code splitting with React.lazy() and Suspense.

All 20 page components were eagerly imported in App.tsx, so the initial bundle included every route. This change code-splits each page into its own chunk loaded on demand.

What changed

• src/utils/lazyWithRetry.ts (new): a React.lazy wrapper that caches the import() promise and clears it on rejection, so a momentary chunk-load failure can be retried on the next render (via the ErrorBoundary reset) instead of permanently poisoning the lazy component.
• src/components/RouteLoader.tsx (new): centered spinner + i18n "Loading…" text used as the Suspense fallback for every lazy route.
• src/components/ErrorBoundary.tsx: now supports a function-form fallback (({ onReset }) => …) and an internal resetErrorBoundary that re-renders children — enabling a working "Try again" button after a chunk-load failure.
• src/App.tsx: all 18 unique page components are now lazy-loaded via lazyWithRetry; each route element is wrapped in <Suspense fallback={<RouteLoader />}> inside an <ErrorBoundary>. The EmployerLayout shell stays eagerly imported so nested-route Suspense never bubbles to the root without a fallback. Route structure is unchanged.

Verification

• npx tsc --noEmit — passes.
• vite build — succeeds, and confirms per-route chunks:

dist/assets/Home-CZxR7r.js              2.8 kB │ gzip:  0.9 kB
dist/assets/PayrollScheduler-…          35.0 kB │ gzip:  9.6 kB
dist/assets/AdminPanel-…                53.4 kB │ gzip: 10.9 kB
dist/assets/EmployeeEntry-…            113.9 kB │ gzip: 40.6 kB
dist/assets/Forecasting-…              354.7 kB │ gzip: 106.1 kB

Each page now loads its own chunk on navigation; the initial index bundle no longer includes all 20 pages.

Acceptance criteria

• Each route loads its chunk on demand (per-route chunks visible in build output)
• Loading spinner shown during chunk load (RouteLoader fallback)
• Error boundary catches chunk load failures with retry (lazyWithRetry clears the promise; ErrorBoundary function fallback exposes working onReset → "Try again")
• Bundle size reduction visible in build output (12+ separate page chunks instead of one app bundle)