F5: Drag Project between Clients

[rororowyourboat]

Summary

Allow users to reassign a Project from one Client to another by dragging it in the sidebar.

Scope

• Extend dnd-kit setup to support cross-container drops (Project dragged onto a different Client section)
• Add useDroppable zones on Client section headers/bodies
• On cross-container drop: dispatch project.client.reassign command
• On drop onto "Unassigned": dispatch project.client.unassign command
• On drag from "Unassigned" onto a Client: dispatch project.client.assign command
• Drop onto same Client: no-op
• Drop onto archived Client: rejected with visual feedback

Technical notes — dnd-kit refactor required

The current drag setup needs significant changes for cross-container support:

1. Multiple SortableContext containers — one per Client section (currently one flat list)
2. DragOverlay component — needed for visual feedback during cross-container drags (currently absent)
3. Custom collision detection — must distinguish "reorder within container" from "move to different container"
4. Conditional modifiers — vertical-only for reorder within a Client, free movement when crossing Clients
5. Data payload on drag items — tag each draggable with { type: "client" | "project", id } to route onDragEnd logic

• Optimistic UI with rollback on command failure

Key files to modify

• apps/web/src/components/Sidebar.tsx — major DndContext refactor: multiple containers, DragOverlay, collision detection
• apps/web/src/store.ts — optimistic project reassignment

Dependencies

• F2 (Client-Project binding commands), F3 (sidebar hierarchy)

Acceptance criteria

• Can drag a Project from one Client section to another
• Drop zone highlights on valid targets
• Drop onto "Unassigned" unassigns the project
• Drag from "Unassigned" onto a Client assigns it
• Drop onto same Client is a no-op
• Drop onto archived Client shows rejection feedback
• Thread items cannot be dropped onto Clients
• Optimistic UI with rollback

[rororowyourboat]

Implemented in feat/sidebar-project-groups branch.