- Drag handle at bottom edge of each module (GripHorizontal icon)
- Pointer capture tracks vertical drag delta → U-size delta
- Clamped to: minimum 1U, rack bounds, first module below
- Shows current U-size label during active resize
- On release: PUT /modules/:id with new uSize (server validates collision)
- Optimistic store update via updateModuleLocal on success
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>