feat(client): add routing and sidebar icons

- install react-router-dom and wrap app in HashRouter to avoid dev proxy conflicts
- replace useState-based section switching with NavLink + Routes
- add lucide icons (Globe, KeyRound, Monitor, ClipboardList) to nav items
- icon opacity transitions on hover and active state via CSS

client/src/main.tsx

@@ -1,11 +1,14 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
+import { HashRouter } from 'react-router-dom'
 import './i18n'
 import './ui/tokens.css'
 import App from './App'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <App />
+    <HashRouter>
+      <App />
+    </HashRouter>
   </StrictMode>,
 )