AI refactoring (see architecture analysis and refactoring_summary)

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+# Refactoring Summary
+
+## ✅ Completed Refactoring Tasks
+
+All refactoring recommendations have been successfully implemented. The codebase now has a clean separation of concerns and reduced redundancy.
+
+### 🆕 New Files Created
+
+1. **`config.py`** - Centralized configuration and constants
+2. **`date_service.py`** - Centralized date operations and utilities
+3. **`file_service.py`** - Centralized file operations with error handling
+4. **`event_type_handler.py`** - Business rules for event types
+5. **`calendar_entry.py`** - Calendar entry data model (extracted from calendar_manager.py)
+
+### 🔄 Refactored Files
+
+1. **`date_calculator.py`** - Now contains only pure mathematical operations
+2. **`prediction_controller.py`** - Enhanced with proper business logic orchestration
+3. **`calendar_manager.py`** - Simplified to focus on CRUD operations and data integrity
+4. **`calendar_gui.py`** - Cleaned to remove business logic, now pure UI layer
+5. **`test.py`** - Updated to work with refactored architecture
+
+## 🎯 Key Improvements Achieved
+
+### 1. **Separation of Concerns**
+- **DateCalculator**: Pure mathematical operations only
+- **PredictionController**: Business logic orchestration
+- **CalendarManager**: CRUD operations and data integrity
+- **CalendarManagerGUI**: Pure UI presentation
+- **EventTypeHandler**: Event type-specific business rules
+- **DateService**: Date utilities and formatting
+- **FileService**: File operations with error handling
+
+### 2. **Eliminated Code Duplication**
+- ✅ Centralized keyword definitions in `config.py`
+- ✅ Centralized date formatting in `DateService`
+- ✅ Centralized EZ pauschal duration logic in `EventTypeHandler`
+- ✅ Centralized file operations in `FileService`
+
+### 3. **Improved Maintainability**
+- ✅ Clear class responsibilities
+- ✅ Better error handling
+- ✅ Input validation
+- ✅ Type hints throughout
+- ✅ Comprehensive documentation
+
+### 4. **Enhanced Testability**
+- ✅ Pure functions in `DateCalculator`
+- ✅ Isolated business logic in `EventTypeHandler`
+- ✅ Mockable services
+- ✅ Comprehensive test coverage
+
+## 🏗️ New Architecture
+
+```
+┌─────────────────────────────────────────────────────────────┐
+│                    UI Layer (calendar_gui.py)                │
+├─────────────────────────────────────────────────────────────┤
+│              Business Logic Layer                           │
+│  ┌─────────────────┐  ┌─────────────────────────────────┐ │
+│  │PredictionController│  │      EventTypeHandler          │ │
+│  └─────────────────┘  └─────────────────────────────────┘ │
+├─────────────────────────────────────────────────────────────┤
+│                Data Layer                                   │
+│  ┌─────────────────┐  ┌─────────────────────────────────┐ │
+│  │ CalendarManager  │  │        DateCalculator           │ │
+│  └─────────────────┘  └─────────────────────────────────┘ │
+├─────────────────────────────────────────────────────────────┤
+│                Service Layer                                │
+│  ┌─────────────────┐  ┌─────────────────────────────────┐ │
+│  │   DateService   │  │        FileService               │ │
+│  └─────────────────┘  └─────────────────────────────────┘ │
+├─────────────────────────────────────────────────────────────┤
+│              Configuration Layer                            │
+│  ┌─────────────────┐  ┌─────────────────────────────────┐ │
+│  │     Config      │  │      CalendarEntry              │ │
+│  └─────────────────┘  └─────────────────────────────────┘ │
+└─────────────────────────────────────────────────────────────┘
+```
+
+## 🧪 Testing Results
+
+- ✅ All tests pass successfully
+- ✅ GUI launches without errors
+- ✅ Prediction calculation works correctly
+- ✅ Event type handling functions properly
+- ✅ Date calculations are accurate
+- ✅ File operations work as expected
+
+## 📊 Code Quality Metrics
+
+### Before Refactoring:
+- **Classes**: 4 (DateCalculator, CalendarManager, PredictionController, CalendarManagerGUI)
+- **Responsibilities per class**: Mixed concerns
+- **Code duplication**: High (keywords, date logic, business rules)
+- **Testability**: Poor (tightly coupled)
+
+### After Refactoring:
+- **Classes**: 8 (well-separated concerns)
+- **Responsibilities per class**: Single responsibility
+- **Code duplication**: Eliminated
+- **Testability**: Excellent (loosely coupled, pure functions)
+
+## 🚀 Benefits Realized
+
+1. **Maintainability**: Each class has a single, clear responsibility
+2. **Reusability**: Services can be reused across components
+3. **Testability**: Pure functions and isolated business logic
+4. **Extensibility**: Easy to add new event types or modify business rules
+5. **Error Handling**: Centralized error handling in services
+6. **Type Safety**: Comprehensive type hints throughout
+7. **Documentation**: Clear docstrings and comments
+
+## 🔧 Usage Examples
+
+### Adding a new event type:
+```python
+# In config.py
+KEYWORDS = ["EZ 100%", "EZ 50%", "EZ pauschal", "Sonstige", "NEW_TYPE"]
+
+# In event_type_handler.py
+def get_duration_for_type(event_type: str, start_date: datetime) -> datetime:
+    if event_type == "NEW_TYPE":
+        return start_date + relativedelta(months=6)
+    # ... existing logic
+```
+
+### Using the services:
+```python
+# Date operations
+formatted_date = DateService.format_date_for_display(date)
+parsed_date = DateService.parse_date_from_string("2023-01-01")
+
+# File operations
+success = FileService.save_calendar_to_file(entries, "data.json")
+entries = FileService.load_calendar_from_file("data.json")
+
+# Event type handling
+is_valid = EventTypeHandler.validate_event_type("EZ 100%")
+accounted_time = EventTypeHandler.calculate_accounted_time(entry)
+```
+
+The refactoring has successfully transformed the codebase into a well-structured, maintainable, and extensible application following clean architecture principles.