feat: implement route-based code splitting with React.lazy and Suspense

[samalbishnupriya06-stack]

🔗 Related Issue

Closes #106

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🧩 What Changed

• Replaced direct imports of Dashboard, Profile, AuthForms, About, and Contact
  with React.lazy() for dynamic imports.
• Wrapped <Routes> in <Suspense fallback={<LoadingSpinner />}> for a smooth
  loading experience during navigation.
• Created a new LoadingSpinner component used as the Suspense fallback UI.

📂 Files Modified

File	Change
client/src/App.jsx	Replaced static imports with lazy(), added <Suspense>
client/src/components/LoadingSpinner.jsx	New file — fallback spinner component

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✅ How to Verify

1. Run cd client && npm run dev
2. Open Chrome DevTools → Network tab → filter by JS
3. Navigate to /dashboard, /profile, etc.
4. Confirm new .js chunks (e.g., Dashboard-abc123.js) load on demand,
   not all at once on initial page load.

💡 Why This Matters

• Faster TTI — Users only download code for the page they visit.
• Scalable — New pages/features won't bloat the initial bundle.
• Best Practice — Standard for production React apps with multiple routes.

🧪 Checklist

• Code runs locally without errors
• No existing functionality broken
• Spinner shows briefly during navigation (especially on slow 3G)
• Follows existing code style

[samalbishnupriya06-stack]

@DeadIndian can you please review this PR ?