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/hooks/use-subscription.ts

@@ -0,0 +1,31 @@
+import { useQuery } from '@tanstack/react-query';
+import { subscriptionApi } from '@/lib/subscription-api';
+
+export const subscriptionKeys = {
+  all: ['subscription'] as const,
+  plans: () => ['subscription', 'plans'] as const,
+  billing: () => ['subscription', 'billing'] as const,
+  quota: (metric: string) => ['subscription', 'quota', metric] as const,
+};
+
+export function usePlans() {
+  return useQuery({
+    queryKey: subscriptionKeys.plans(),
+    queryFn: () => subscriptionApi.getPlans(),
+  });
+}
+
+export function useBillingHistory() {
+  return useQuery({
+    queryKey: subscriptionKeys.billing(),
+    queryFn: () => subscriptionApi.getBillingHistory(),
+  });
+}
+
+export function useQuota(metric: string) {
+  return useQuery({
+    queryKey: subscriptionKeys.quota(metric),
+    queryFn: () => subscriptionApi.checkQuota(metric),
+    enabled: !!metric,
+  });
+}