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/main.py

@@ -408,6 +408,9 @@ def _run_migrations():
         )""",
         # Printer duplicate copies (0 = print once, 1 = print twice, etc.)
         "ALTER TABLE printers ADD COLUMN duplicates INTEGER NOT NULL DEFAULT 0",
+        # Waiter cancellation permissions
+        "ALTER TABLE users ADD COLUMN can_cancel_orders INTEGER NOT NULL DEFAULT 0",
+        "INSERT OR IGNORE INTO pos_settings (key, value, updated_at) VALUES ('orders.waiter_cancellations_allowed', 'false', CURRENT_TIMESTAMP)",
     ]
     for sql in migrations:
         try: