iOS thumbnails: add batch-thumbs command for future optimization
batch-thumbs pulls multiple files in one Python process session, amortising the pymobiledevice3 startup cost. Not yet wired into the app — thumbnails load lazily per-cell and cache permanently. Known limitation: first load of iPhone thumbnails is slow (~2-3s each) because each pull spawns a new Python process. After first view they are instant from the on-disk cache.
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