chore: remove legacy @zxing packages and old decoder implementation

Remove @zxing/browser and @zxing/library dependencies as they have been fully replaced by zxing-wasm. Delete old barcodeDecoder.ts file which is no longer used after migration to barcodeDecoderWasm.ts.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

package.json

@@ -17,8 +17,6 @@
     "@tailwindcss/typography": "^0.5.10",
     "@types/iban": "^0.0.35",
     "@types/node": "20.5.7",
-    "@zxing/browser": "^0.1.5",
-    "@zxing/library": "^0.21.3",
     "autoprefixer": "10.4.15",
     "bcrypt": "^5.1.1",
     "clsx": "^2.0.0",