- Add google_mlkit_barcode_scanning dependency
- Scan priority: Barcode → OCR text → Nothing found
- Support UPC-A/E, EAN-8/13, QR, DataMatrix, PDF417, Code128/39
- BarcodeData class with product code detection and NDC hints
- ScanResult with type indicator (barcode|text|empty)
- Updated capture screen with barcode-specific UI:
- Green overlay for barcodes, blue for text
- Product code badge for UPC/EAN barcodes
- Format-specific result display
- Backward compatible legacy API (extractText, getTextBlocks)