Clear the component hover outline when a drag starts

Overview

In the Canvas editor, the hover outline on a component in the preview stays visible after you start dragging it, and is still there after the drag ends, after an invalid drop, and after cancelling with Escape.

To reproduce: hover a component in the preview, press the mouse down, and move a small distance without leaving that component's bounds.

The editor stores a single hovered component uuid, written on mouseover and cleared on mouseout. The drag sensor activates after 3px, so a normal drag starts without the pointer crossing the component's boundary, no mouseout fires, and the stale uuid survives. It only clears when the pointer next crosses some overlay boundary, which makes the bug look intermittent.

Proposed Resolution

Clear the hovered component in the drag start handler in ui/src/features/layout/previewOverlay/DragEventsHandler.tsx, which sees every drag start regardless of origin. The layers panel already does this; the preview overlay had no equivalent. Add Playwright coverage for a drag that stays inside the component's bounds.