fix: AI visual barcode fallback fires only once per scanner session

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.

translations/de.json

@@ -223,7 +223,8 @@
     "status_parallel": "Kombinierter Scan aktiv...",
     "ai_fallback_searching": "KI identifiziert Produkt...",
     "ai_fallback_found": "Produkt von KI erkannt",
-    "ai_fallback_not_found": "KI: Produkt nicht erkannt"
+    "ai_fallback_not_found": "KI: Produkt nicht erkannt",
+    "ai_fallback_exhausted": "KI: Produkt nicht erkannt — Barcode erneut scannen"
   },
   "action": {
     "title": "Was möchtest du tun?",