feat(plans): menu watermark on Free (removed by paid feature)

Guest QR menu shows a "ساخته‌شده با میزی" watermark under the menu unless the café's
plan has the `watermark_removed` feature (Starter+).

- PublicMenuDto gains ShowWatermark; PublicService computes it from
  IsFeatureEnabledForCafeAsync("watermark_removed") for both slug and branch menus.
- Guest menu renders the watermark footer when showWatermark.
- NoOpPlatformCatalogService test double (all features on) for the PublicService
  ctor; QrMenuTests updated.

86 tests pass; dashboard tsc clean.

web/dashboard/src/lib/api/qr-public.ts

@@ -53,6 +53,8 @@ export type QrPublicMenu = {
   slug: string;
   theme?: CafeTheme | null;
   categories: QrPublicMenuCategory[];
+  /** Free plan shows the Meezi watermark under the menu; paid plans hide it. */
+  showWatermark?: boolean;
 };
 
 export type QrCartLine = {