chore: remove shadcn components/ui dir; migrate login+topbar+providers to native components; single design system

components/providers.tsx

@@ -2,7 +2,7 @@
 
 import { useState } from 'react';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
-import { Toaster } from '@/components/ui/sonner';
+import { Toaster } from 'sonner';
 
 export default function Providers({ children }: { children: React.ReactNode }) {
   // Must be created inside component — NOT at module level