feat: preview and check large PNG sheets by reading them as a stream

A large sheet had no preview and no automatic check of pieces, residue,
gaps and background, because decoding it whole would crash the browser.
A PNG over 150 MB is now inflated as a stream (DecompressionStream) and
unfiltered row by row, keeping every n-th pixel: a 600-pixel-wide copy in
about 100 MB of memory whatever the file's size (6.6 s for 500 MB, 22.7 s
for 2.4 GB of random pixels, identical to Pillow's output for RGB, RGBA,
grey, grey+alpha, palette and 16-bit). The copy is the preview and what
the sheet check reads; the grade still comes at once from the header.
The preview note promising a server-side re-nesting that does not exist
is removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/index.html

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