feat: AI visual barcode fallback after 5s with settings toggle

When the barcode scanner cannot read a code within 5 seconds and Gemini
is available, a camera frame is automatically captured and sent to the
new gemini_barcode_visual endpoint for visual product identification.
The result pre-fills the product form identically to a barcode scan.

- PHP: new geminiBarcodeVisual() function + router case + aiActions entry
- PHP: barcode_ai_fallback setting in getServerSettings() + saveSettings() boolMap
- JS: _aiFallbackTimer (cleared on detection/stop), 5s timer in initScanner()
- JS: _tryGeminiVisualBarcode() — captures JPEG frame, calls API, saves product
- JS: barcode_ai_fallback wired into serverKeys, applyUI, collectUI, POST body
- HTML: AI fallback toggle in Settings → Camera card
- Translations: ai_fallback_* strings in scan + settings.camera (it/en/de/fr/es)

Feature is disabled by default (BARCODE_AI_FALLBACK=false).

CHANGELOG.md

@@ -11,6 +11,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
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+## [1.7.34] - 2026-05-30
+
+### Added
+- **AI visual barcode fallback** — When the barcode scanner fails to read a barcode within 5 seconds, EverShelf can now automatically capture a camera frame and send it to Gemini Vision to visually identify the product (name, brand, category). On success the product is saved and the inventory form opens just as if a barcode had been scanned. A new toggle in **Settings → Camera** (`AI visual identification (5s fallback)`) lets users enable or disable this feature at any time. Requires Gemini API key configured. Disabled by default.
+
 ## [1.7.33] - 2026-05-29
 
 ### Fixed