Add DueDate functionality to TaskHub tasks

[Copilot]

This PR adds the ability to set and manage due dates for tasks in TaskHub, addressing the feature request "Kann ich bei Tasks auch ein DueDate hinterlegen?" (Can I also store a DueDate for tasks?).

Features Added

✅ Task Creation with Due Dates: Users can now add optional due dates when creating new tasks
✅ Due Date Display: Tasks show due dates in a readable format (e.g., "Due: 12/25/2024")
✅ Due Date Editing: Double-click on due dates to edit them inline
✅ Add Due Date to Existing Tasks: Button to add due dates to tasks that don't have them
✅ Overdue Detection: Visual styling for overdue tasks (red background)
✅ Form Integration: Date input field integrated into the add task form
✅ Data Persistence: Due dates are stored with the task data

Screenshot

The screenshot shows:

• New date input field in the task creation form
• Due dates displayed on tasks (e.g., "Due: 12/25/2024")
• Visual distinction for overdue tasks
• "+ Add Due Date" button for tasks without due dates

Technical Implementation

Changes Made:

• TodoTypes.ts: Added dueDate?: Date to Todo interface and extended TodoContextType
• TodoContext.tsx: Added updateTodoDueDate function and modified addTodo to accept due dates
• AddTodoForm: Added date input field for setting due dates during task creation
• TodoItem: Enhanced with due date display, editing, and overdue detection
• TodoList: Updated to pass due date update function to TodoItem components
• Styling: Added comprehensive CSS for due date elements with hover effects and overdue styling

Key Features:

• Optional Due Dates: Due dates are completely optional - tasks work with or without them
• Inline Editing: Double-click any due date to edit it directly
• Overdue Detection: Automatically detects and highlights overdue tasks
• Consistent UX: Follows the existing double-click-to-edit pattern used for task text and time
• Data Persistence: Due dates are saved and loaded with the task file

Testing

The implementation has been thoroughly tested:

• ✅ Creating tasks with due dates
• ✅ Creating tasks without due dates
• ✅ Adding due dates to existing tasks
• ✅ Editing existing due dates
• ✅ Overdue date detection and styling
• ✅ Form clearing after task creation
• ✅ Build and compile successfully

All existing functionality remains intact - this is a purely additive feature that doesn't break any existing workflows.

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