POS: Block barcode scanning on error popup
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Summary
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This Point of Sale extension prevents the POS from processing further barcode
scans while the "barcode not found" error popup is visible. It also avoids the
scanner's Enter/Escape keystrokes from closing the popup, so the cashier must
explicitly dismiss it (click/tap) before continuing.

Features
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- Takes exclusive control of the barcode reader while the error popup is shown.
- Prevents scanner-triggered keyboard "clicks" from closing the popup.
- Keeps the initial error message visible; subsequent scans are ignored.
- Includes CSS to highlight the error popup for better visibility.

Usage
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- Scan a non-existent barcode in the POS.
  - The error popup will show.
  - Further scans will be ignored while the popup is visible.
  - Close the popup via mouse/touch to resume scanning.

Limitations
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- While the popup is open, all barcode actions are blocked (product, client,
  weight, price, discount, GS1, etc.). Customize the hook if you need
  exceptions.


Credits
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- Author: Criptomart
- License: AGPL-3