If Gemini cannot identify the product visually, mark _aiFallbackExhausted=true
for the current scanner session so the 5s timer never fires again. The scanner
restarts normally (user can keep trying with the barcode reader) and a persistent
status message is shown: 'AI: product not recognized — try scanning the barcode'.
_aiFallbackExhausted resets to false in stopScanner() so the next camera session
starts fresh.
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