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/App.module.css

@@ -29,7 +29,9 @@
 }
 
 .navItem {
-  display: block;
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
   width: 100%;
   padding: var(--space-2) var(--space-3);
   background: none;
@@ -40,9 +42,21 @@
   font-family: var(--font-family);
   text-align: left;
   cursor: pointer;
+  text-decoration: none;
   transition: background 120ms ease, color 120ms ease;
 }
 
+.navIcon {
+  flex-shrink: 0;
+  opacity: 0.65;
+  transition: opacity 120ms ease;
+}
+
+.navItem:hover .navIcon,
+.navItemActive .navIcon {
+  opacity: 1;
+}
+
 .navItem:hover {
   background: var(--color-bg-subtle);
 }