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.

src/Meezi.API/Models/Public/PublicDtos.cs

@@ -107,7 +107,8 @@ public record PublicMenuDto(
     string CafeName,
     string Slug,
     CafeThemeDto Theme,
-    IReadOnlyList<PublicMenuCategoryDto> Categories);
+    IReadOnlyList<PublicMenuCategoryDto> Categories,
+    bool ShowWatermark);
 
 public record GuestCreateOrderRequest(
     OrderType OrderType,