feat: major dashboard & waiter PWA overhaul

- Manager dashboard: replaced monolithic DashboardTab/OperationsPage with new
  DashboardPage; added OrderDetailModal, ShiftDetailModal, DeleteConfirmModal,
  PaymentMethodModal; updated Sidebar routing and App navigation
- Reports: reworked WorkDaySummary, OrderHistory, ShiftsOverview with detail modals
- Backend routers: extended orders, reports, shifts, products, business_day endpoints;
  updated cloud_sync service
- Waiter PWA: refreshed app icons, improved ConnectionLostModal UX, updated
  TableCard, SSEContext, connectionStore; added useProductCache hook; vite config tweaks

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

local_backend/routers/business_day.py

@@ -162,6 +162,33 @@ def close_business_day(
     return day
 
 
+@router.delete("/{day_id}", status_code=status.HTTP_204_NO_CONTENT)
+def delete_business_day(
+    day_id: int,
+    db: Session = Depends(get_db),
+    user: User = Depends(require_manager),
+):
+    """Permanently delete a business day. Only allowed when it has no shifts."""
+    day = db.query(BusinessDay).filter(BusinessDay.id == day_id).first()
+    if not day:
+        raise HTTPException(status_code=404, detail="Business day not found")
+    if day.status == "open":
+        raise HTTPException(status_code=400, detail="Cannot delete an open business day — close it first")
+
+    shift_count = db.query(WaiterShift).filter(WaiterShift.business_day_id == day_id).count()
+    if shift_count > 0:
+        raise HTTPException(
+            status_code=409,
+            detail=f"Δεν είναι δυνατή η διαγραφή: η εργάσιμη μέρα έχει {shift_count} βάρδια/ες. Διαγράψτε πρώτα όλες τις βάρδιες.",
+        )
+
+    db.query(Order).filter(Order.business_day_id == day_id).update(
+        {"business_day_id": None}, synchronize_session=False
+    )
+    db.delete(day)
+    db.commit()
+
+
 @router.get("/history")
 def business_day_history(
     db: Session = Depends(get_db),