feat: Feature 2 — waiter order cancellation + report updates

Backend:
- Add can_cancel_orders to User model and schema
- Add global orders.waiter_cancellations_allowed setting (migration)
- Cancel endpoints: mark items with cancelled_by/cancelled_at, fire cancellation print
- print_cancellation_ticket: routes to same printer zones, prints ΑΚΥΡΩΣΗ banner
- Fix cancellations_log: date filter, waiter filter, join syntax
- shift/orders: add cancellations count and hours_worked per waiter
- _enrich_shift: add cancellations count to shift data
- Add cancel-permissions endpoint for PWA

Manager dashboard:
- Global cancel setting toggle in Settings > Operation > Shift Settings
- Per-waiter can_cancel_orders checkbox in staff modal
- Manager cancel flow: print confirmation prompt (Ναι/Όχι) in DashboardPage
- ShiftsOverview: Ακυρώσεις column per shift
- Activity: multi-bar chart with ORDERS/ITEMS/CANCELLATIONS/ΕΣΟΔΑ/ΩΡΕΣ checkboxes,
  grouped/stacked switch, right X-axis for hours, full waiter name on hover
- OrderHistory: cancelled items count column per order
- WorkDaySummary drill-down: cancelled items column in orders tab

Waiter PWA:
- Replace 3 pills with CLEAR | ALL | ACTIONS
- ACTIONS opens ItemActionModal for selected items
- ItemActionModal: ORDER AGAIN, MOVE TO OTHER TABLE, SPLIT, CANCEL ORDER
- ActionsSheet: Cancel Παραγγελίας option (greyed if no permission)
- CancelConfirmModal: requires confirmation before cancelling
- TableListPage: cancel order from long-press quick modal

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

local_backend/models/user.py

@@ -26,6 +26,8 @@ class User(Base):
     created_at = Column(DateTime(timezone=True), default=_utcnow)
     # Phase 2B — payroll
     hourly_rate = Column(Float, nullable=True)
+    # Waiter cancellation permission
+    can_cancel_orders = Column(Boolean, default=False, nullable=False)
 
     orders_opened = relationship("Order", foreign_keys="Order.opened_by", back_populates="opener")
     orders_closed = relationship("Order", foreign_keys="Order.closed_by", back_populates="closer")