Return nil from getSharedImageData() when the current sharedPhotoShareId
equals the stored sharedPhotoProcessedShareId, so a share that was already
marked processed is no longer re-imported on subsequent app activations.
The check runs before reading the image file and logs
"[ShareTarget] shareId=<id> already processed; skipping". It leaves the
image file and all metadata intact (cleanup is deferred to a later phase),
and does not change behavior for brand-new shares. iOS only; no changes to
markProcessed(), the Share Extension, Android, or the JS flow.
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