web: Fix off-by-one bug in page switching logic
web: Fix off-by-one bug in page switching logic

nafraf/spreads,adongy/spreads,nafraf/spreads,miloh/spreads,miloh/spreads,adongy/spreads,gareth8118/spreads,gareth8118/spreads,adongy/spreads,DIYBookScanner/spreads,nafraf/spreads,gareth8118/spreads,DIYBookScanner/spreads,miloh/spreads,DIYBookScanner/spreads