feat(orders): per-item kitchen/bar notes (POS + QR app + KDS)

Lets the POS agent and the QR/app customer attach a free-text note to each
order line (e.g. "no tomato", "extra hot") that reaches the kitchen/bar.

- Backend already supported it (OrderItem.Notes persists; CreateOrderItemRequest
  and OrderItemDto carry Notes; LiveOrderDto items include it) — this wires the UI.
- cart.store: add setNotes(menuItemId, notes); notes already travel in
  getPendingLines and round-trip via hydrateFromOrder.
- POS pos-screen: a note input under each cart line.
- QR guest menu: a note input under each cart line (QrCartLine.note).
- KDS: render the note prominently under each item so kitchen/bar sees it.
- i18n: pos.itemNotePlaceholder + qrMenu.itemNote (fa/ar/en).

Note: notes are captured on items being added; editing a note on an
already-submitted line is out of scope (no pending delta to re-send).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/dashboard/messages/ar.json

@@ -270,6 +270,7 @@
     "void": "إلغاء",
     "voidItem": "إلغاء الصنف",
     "voided": "ملغى",
+    "itemNotePlaceholder": "ملاحظة للمطبخ/البار (اختياري)",
     "confirmVoid": "هل أنت متأكد أنك تريد إلغاء هذا الصنف؟",
     "voidError": "تعذر إلغاء الصنف",
     "transferTable": "نقل الطاولة",
@@ -883,6 +884,7 @@
     "orderHint": "سيقوم الموظفون بتحضير طلبك قريباً",
     "guestName": "اسمك (اختياري)",
     "guestPhone": "الجوال (اختياري)",
+    "itemNote": "ملاحظة (مثلاً بدون طماطم، سكر أقل)",
     "addMoreItems": "إضافة المزيد",
     "orderError": "تعذر تسجيل الطلب. حاول مرة أخرى.",
     "rateLimited": "طلبات كثيرة — انتظر بضع دقائق",