feat(theme-studio): warn which ramp steps collide with existing names

A ramp step whose name already exists in the palette is skipped on add, but the only signal was a count. Now preview marks each colliding tile with a dashed outline and a badge, and the message names every collision, so you can see which steps won't add before you add them. Add-all reports the skipped names too, not just how many. The single-tile add already named its one collision; this extends the same warning to preview and add-all.

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