Keep a detection's box corners editable while editing its line

[mattdawkins]

• In line editing mode, when the detection also has a box and boxes are visible, the box corners get drag handles too, so the auto-generated box can be adjusted without leaving line mode.
• Implemented as a second EditAnnotationLayer (companion: true, corner handles only) that is live whenever the selected track has both a saved line at the selected key and bounds; it stays off while the line is still being drawn.
• GeoJS applies a handle drag to every annotation layer in edit mode, so each layer records at actiondown whether its own handle was grabbed and only that one moves and commits. A box commit does not rebuild the line layer, and a hovered handle's drag action is restored after the peer layer changes mode.
• Covered by a real-GeoJS jsdom spec that drags a box corner and a line vertex with the other layer's handle previously hovered.

🤖 Generated with Claude Code