Feature: pinnable overlay panels for edge groups (VS-style auto-hide)

[mathuo]

Follow-up from #664.

Edge groups (https://dockview.dev/docs/core/groups/edgeGroups/) cover most of the collapsible-group request, but one piece is still missing: a pinnable / auto-hide overlay mode, like Visual Studio's tool windows.

Desired behavior

• A group docked to an edge can be "unpinned", collapsing it to a strip of tab labels along that edge.
• Clicking a tab label slides the group out as a temporary overlay on top of the existing layout (does not displace neighbouring panels).
• Clicking elsewhere — or selecting another tab — dismisses the overlay.
• A pin toggle on the overlay returns the group to its normal docked state.

Reference

See the screen recording in #664 for the interaction model the original requester was asking for.

Notes

• Edge groups already provide the structural docking position; this issue is specifically about the overlay/auto-hide visual mode on top of that.