feat: demo data seeder — one-click setup for new cafés

Adds POST /api/cafes/{cafeId}/demo/seed (owner-only) that seeds:
- 9% default VAT tax
- 7 menu categories + 59+ items via DemoMenuSeeder
- 15 inventory ingredients (coffee shop staples)
- 10 tables across 3 floors on the first active branch

Frontend DemoDataBanner appears on menu, tables, and inventory
pages when the café is completely empty, so owners can populate
demo data in one click instead of entering everything manually.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/dashboard/src/components/menu/menu-admin-screen.tsx

@@ -5,6 +5,7 @@ import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { useLocale, useTranslations } from "next-intl";
 import { useIsRtl } from "@/lib/use-is-rtl";
 import { Box, Pencil, Plus, Search, Video, X } from "lucide-react";
+import { DemoDataBanner } from "@/components/demo/demo-data-banner";
 import { Menu3dUpload } from "@/components/media/menu-3d-upload";
 import { MenuAi3dGenerate } from "@/components/media/menu-ai-3d-generate";
 import { CategoryVisual } from "@/components/menu/category-visual";
@@ -449,7 +450,19 @@ export function MenuAdminScreen() {
         )
       ) : (
         /* ── Catalog tab ─────────────────────────────────────────────────── */
-        <div className="flex min-h-0 gap-4">
+        <div className="flex min-h-0 flex-col gap-4">
+          {categories.length === 0 && items.length === 0 && (
+            <DemoDataBanner
+              invalidateKeys={[
+                ["menu-categories", cafeId],
+                ["menu-items-all", cafeId],
+                ["menu-items", cafeId],
+                ["tables-board", cafeId],
+                ["inventory", cafeId],
+              ]}
+            />
+          )}
+          <div className="flex min-h-0 gap-4">
 
           {/* ── Category Sidebar (desktop) ─────────────────────────────── */}
           <aside className="hidden w-52 shrink-0 lg:block">
@@ -752,6 +765,7 @@ export function MenuAdminScreen() {
               </div>
             )}
           </div>
+          </div>
         </div>
       )}