Canteen Management

Done. Printable tray label at /dietary/trays/{id}/label (window.print()
on load, Code128 barcode of the tray number, allergy section built on a
pure allergyStateFor helper with four distinct states -- not-plated,
none, listed, unreadable -- malformed data reads as a loud warning, never
silently as "no allergies"). The label's printed link carries ?trayId=;
tray-round now reads it and jumps straight to that tray via a new
findIndexByTrayId helper.

Added GET /api/canteen/dietary/trays/{id} + extended TrayDto with
ServiceDate/MealSlotId/DietTypeCode, needed by both tasks.

Backend: 785/787 (2 pre-existing unrelated failures). Portal: type-check
clean, 480/480 tests green. Pushed to integration/canteen-v1 (9a246f0).