Feature: Space grouping in rail with drag and drop

[mjkatgithub]

Goal

Group spaces in the space rail and support drag-and-drop reordering.

Scope

• Introduce grouped rendering in the space rail
• Reorder spaces within and across groups
• Persist grouping and order (local persistence for MVP)

Acceptance Criteria

• Space groups are visible in the space rail
• Spaces can be reordered intuitively via drag and drop
• Grouping and order are preserved after reload

Test Checklist

Unit

• Group model transformation logic
• Reorder and persistence serialization logic

Integration

• Space rail updates correctly after drag and drop
• Persisted grouping/order is restored correctly

E2E

• Move a space between groups
• Reload and verify persisted grouping/order

Branch

feature/space-grouping-dnd