Persist scalebar detection overlay on first image in detail view

[JonaRichter]

After running µm/px auto-detect, the scalebar frame and px/distance annotation are shown as a preview but not attached to the first image result. Navigating away and back loses the overlay.

Expected: The scalebar detection overlay (bounding box + px/µm label) is stored on the first image result and visible whenever that image is shown in detail view — same as any other overlay.
Current: Overlay is shown once as a temporary preview; navigating away discards it.