refactor(admin): use named routes for admin index navigation

Replace Referer-based redirects in Admin Index back() and switchUser()
with named routes (admin.index and dashboard) for consistent navigation
behavior independent of the request header.

Add tests verifying back() returns to admin.index, switchUser routes to
the dashboard, and the Referer header is no longer consulted.

app/Livewire/Admin/Index.php

@@ -37,7 +37,7 @@ class Index extends Component
             Auth::login($user);
             refreshSession($team_to_switch_to);
 
-            return redirect(request()->header('Referer'));
+            return redirect()->route('admin.index');
         }
     }
 
@@ -70,7 +70,7 @@ class Index extends Component
         Auth::login($user);
         refreshSession($team_to_switch_to);
 
-        return redirect(request()->header('Referer'));
+        return redirect()->route('dashboard');
     }
 
     private function authorizeAdminAccess(): void