Scaffold arrangement demo slice with CompareBar wiring

Add a self-contained demo build target for the Providers → Packages →
Comparison flow, deployable as a static SPA at /arrangement/.

- vite.demo.config.ts: per-slice build via --mode, base path flips for
  dev vs prod, output to dist-demo/<slice>/
- src/demo/: shared fixtures (7 providers across verified/tier3/tier2
  with real venue photography from brandassets) + Zustand basket store
  with ?compare= URL persistence
- Verified-provider packages now share the nine canonical Essentials
  line items per FA convention; only Optionals/Extras vary
- App-level CompareBar surfaces "Already added" / "Maximum 3" feedback
  via transient store error
- ProviderCard logo objectFit cover→contain so wide logos don't crop
- npm scripts demo:dev / demo:build, deps zustand + react-router-dom

src/demo/apps/arrangement/routes/Providers.tsx

@@ -0,0 +1,38 @@
+import { useState } from 'react';
+import { useNavigate } from 'react-router-dom';
+import {
+  ProvidersStep,
+  EMPTY_FILTER_VALUES,
+  type ProviderFilterValues,
+  type ProviderSortBy,
+  type ListViewMode,
+} from '../../../../components/pages/ProvidersStep';
+import { providers } from '../../../shared/fixtures/providers';
+import { demoNav } from '../DemoNav';
+
+export function ProvidersRoute() {
+  const navigate = useNavigate();
+  const [query, setQuery] = useState('');
+  const [filters, setFilters] = useState<ProviderFilterValues>(EMPTY_FILTER_VALUES);
+  const [sort, setSort] = useState<ProviderSortBy>('recommended');
+  const [view, setView] = useState<ListViewMode>('list');
+
+  const filtered = providers.filter((p) => p.location.toLowerCase().includes(query.toLowerCase()));
+
+  return (
+    <ProvidersStep
+      providers={filtered}
+      onSelectProvider={(id) => navigate(`/providers/${id}/packages`)}
+      searchQuery={query}
+      onSearchChange={setQuery}
+      filterValues={filters}
+      onFilterChange={setFilters}
+      sortBy={sort}
+      onSortChange={setSort}
+      viewMode={view}
+      onViewModeChange={setView}
+      onBack={() => window.history.back()}
+      navigation={demoNav}
+    />
+  );
+}