feat(theme-selector): click-to-move arrows for palette tiles

Dragging the palette tiles with native HTML5 drag was fiddly: small drop targets, no feedback, and the name field inside each tile fought the grab. I added small move-left and move-right arrows on each tile's bottom corners, so reordering is one click and needs no dragging. The first tile drops its left arrow and the last drops its right. Drag still works for anyone who prefers it, now with a dashed outline marking the drop target while you drag.

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