Add drag-and-drop task reordering

[zquintana]

Implements drag-and-drop functionality for reordering tasks with the following features:

• Global ordering system allowing tasks to be positioned anywhere in the list
• Toggle between custom order and automatic sorting
• Smooth animations with DragOverlay
• Visual feedback during drag operations
• Priority remains unchanged when reordering (only position changes)

Changes

• Added @dnd-kit packages for drag-and-drop functionality
• Added optional order field to Task interface for custom positioning
• Added Redux actions for task reordering (reorderTasks, moveTaskToPriority, setTaskOrder, reorderMultipleTasks, clearTaskOrder)
• Updated API service to handle order persistence and migration
• Made TaskItem draggable with visual feedback
• Implemented DndContext and SortableContext in TaskList with toggle button
• Added DragOverlay for smooth drag animations

Closes #7