Make HUD panels draggable from any non-interactive area

Drag a panel from anywhere on its surface (header, padding, labels,
background) instead of only the header. Dragging is cancelled when the
press starts on an interactable (Selectable toggle/slider or the color
picker's UiPointerSurface), so widgets stay fully operable. Clamping is
now anchor/pivot-agnostic, computed from the panel's real corners, so
panels anchored bottom-center stay reachable in every screen corner.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/DraggablePanel.cs.meta

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