Adds a "Search within image" button to the lightbox that lets the user
draw a crop region on an image and find visually similar results using
that crop's embedding. The crop is embedded in-memory without a temp
file via a new embed_image_crop method on ClipImageEmbedder.
Introduces a folder-scoped cosine search (search_image_ids_by_embedding_in_folder)
to support the current-folder scope option, and wires up the new
find_similar_by_region Tauri command end-to-end from Rust through to
the Zustand store and Lightbox UI.
LyAhn
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