feat(web): add React Query, dark mode toggle, and error retry UX

- Install @tanstack/react-query with exponential backoff retry config
- Create QueryClientProvider and custom hooks for listings, analytics,
  payments, and subscription API calls
- Migrate 5 dashboard pages from useState/useEffect to React Query hooks
- Add dark mode CSS variables and ThemeProvider with localStorage persistence
- Add theme toggle button in dashboard header (sun/moon icon)
- Enhance error boundaries with auto-retry, retry count, and loading state

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/lib/query-client.ts

@@ -0,0 +1,32 @@
+'use client';
+
+import { QueryClient } from '@tanstack/react-query';
+
+function makeQueryClient() {
+  return new QueryClient({
+    defaultOptions: {
+      queries: {
+        staleTime: 60 * 1000,
+        gcTime: 5 * 60 * 1000,
+        retry: 3,
+        retryDelay: (attemptIndex) => Math.min(1000 * 2 ** attemptIndex, 30000),
+        refetchOnWindowFocus: false,
+      },
+      mutations: {
+        retry: 1,
+      },
+    },
+  });
+}
+
+let browserQueryClient: QueryClient | undefined;
+
+export function getQueryClient() {
+  if (typeof window === 'undefined') {
+    return makeQueryClient();
+  }
+  if (!browserQueryClient) {
+    browserQueryClient = makeQueryClient();
+  }
+  return browserQueryClient;
+}