Adapt Gantt View for crop planning (hierarchy, grow + harvest periods, bed reassignment)

[stipsitzm]

OpenFarmPlanner currently uses a Gantt view based on React Modern Gantt
https://github.com/MikaStiebitz/React-Modern-Gantt

For real-world CSA and vegetable farm planning, the current flat Gantt view is not sufficient.
This issue describes the required functional extensions to support agricultural planning workflows.

Required changes

1. Hierarchical structure in the left column

The left column must support a hierarchy instead of a flat list.

Target structure:

• Field (Schlag)
  • Bed (Beet)
    • Crop / planting plan

Details:

• Fields and beds are grouping rows and do not render timeline bars.
• Crop rows render the actual timeline bars.
• Fields and beds should be expandable and collapsible.
• Internally, rows may still be handled as a flattened list with metadata such as:
  parentId, depth, isExpanded