Add scanner-aware product search and barcode validation

[IanTheMitten]

Motivation

• Improve cashier workflow by supporting keyboard-wedge barcode scanners that emit a rapid key burst followed by Enter and reduce manual steps when a barcode uniquely identifies a product.
• Prevent invalid or duplicate barcodes at creation time to reduce inventory data errors and provide immediate feedback to the user.

Description

• Implemented scanner-aware input handling in src/components/cashier/ProductSearch.tsx by tracking recent key timestamps and treating Enter as a scanner submit when a rapid-burst pattern is detected; on a single exact barcode match it calls onAddToCart(product) and clears the input, otherwise shows an inline error and a toast while preserving the input.
• Updated cashier search placeholder to "Scan barcode here or type manually." and added visible error styling for search failures.
• Extended src/components/inventory/AddProductModal.tsx with barcode validation including configurable allowed characters via VITE_BARCODE_CHARSET_REGEX, optional length enforcement via VITE_BARCODE_ENFORCE_LENGTH and VITE_BARCODE_ALLOWED_LENGTHS, input sanitization, inline error messages, and pre-submit duplicate checks against in-memory products; server-side barcode conflict errors are handled with a clearer toast fallback.
• Wired the current product list into the modal by adding an existingProducts prop and passing products from src/components/inventory/InventoryPage.tsx so client-side duplicate checks can run before calling productsAPI.create.

Testing

• Built the production bundle with npm run build, which completed successfully.
• Local compile/build checks passed (no runtime build errors reported).

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Codex Task