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/schemas/user.py

@@ -16,6 +16,7 @@ class UserBase(BaseModel):
     email: Optional[str] = None
     # Phase 2B — payroll
     hourly_rate: Optional[float] = None
+    can_cancel_orders: bool = False
 
 
 class UserCreate(UserBase):
@@ -33,6 +34,7 @@ class UserUpdate(BaseModel):
     note: Optional[str] = None
     # Phase 2B — payroll
     hourly_rate: Optional[float] = None
+    can_cancel_orders: Optional[bool] = None
 
 
 class WaiterZoneOut(BaseModel):