refactor(router): move settings route under dashboard

- Settings is now a sub-route
- Improves navigation structure

Author: fulleni

lib/router/router.dart

@@ -140,6 +140,17 @@ GoRouter createRouter({
                 path: Routes.dashboard,
                 name: Routes.dashboardName,
                 builder: (context, state) => const DashboardPage(),
+                routes: [
+                  // The settings page is a sub-route of the dashboard.
+                  // This allows it to be displayed within the AppShell
+                  // (with sidebar and top bar visible) without adding
+                  // a new item to the main navigation sidebar.
+                  GoRoute(
+                    path: Routes.settings,
+                    name: Routes.settingsName,
+                    builder: (context, state) => const SettingsPage(),
+                  ),
+                ],
               ),
             ],
           ),
@@ -178,15 +189,6 @@ GoRouter createRouter({
               ),
             ],
           ),
-          StatefulShellBranch(
-            routes: [
-              GoRoute(
-                path: Routes.settings,
-                name: Routes.settingsName,
-                builder: (context, state) => const SettingsPage(),
-              ),
-            ],
-          ),
         ],
       ),
     ],