Chestny Znak scanner: bundle offline ZXing decoder, ЧЗ badge, session auto-push

- Native BarcodeDetector needs Google Play Services (fails "Barcode detection
  service unavailable" on de-Googled ROMs). Replace it with a bundled, fully
  offline ZXing decoder (frontend/lib/zxing.js, @zxing/library UMD): scanner uses
  BrowserDatamatrixCodeReader (TRY_HARDER) for live camera and decodeFromImageUrl
  for the photo fallback. No Google, codes never leave the device.
- Source badge "ЧЗ" (src-chzn) for products added via the scanner.
- tools/chzn_export.sh: adb push the exported session to the device Downloads so
  BalDav's file picker can import it directly.

frontend/css/styles.css

@@ -1649,6 +1649,7 @@ select {
 .src-kdv { background: rgba(255, 167, 38, 0.25); color: #ffb74d; }
 .src-ion { background: rgba(158, 158, 158, 0.2); color: #9e9e9e; }
 .src-manual { background: rgba(255, 235, 59, 0.2); color: #fff176; }
+.src-chzn { background: rgba(66, 165, 245, 0.22); color: #64b5f6; }
 
 .food-nutrients {
   font-size: 11px;