Implement professional PDF-based label printing for boxes and locations

- Replace ZPL/image-based printing with ReportLab PDF generation
- Box labels: 8cm x 5cm landscape with 'BOX Nr:' header and CODE128 barcode
- Location labels: 8cm x 5cm landscape with 'Location nr:' header and CODE128 barcode
- Add /generate_box_label_pdf endpoint using same approach as print_module
- Update FG scan quick box creation to use PDF printing with default printer
- Switch from CDN QZ Tray to local patched version for pairing-key auth
- Improve error handling and logging throughout printing workflow
- Fix import issues (add mm unit to warehouse.py)
- Optimize barcode size and spacing for better readability

py_app/app/templates/create_locations.html

@@ -510,11 +510,13 @@ async function printLocationBarcode() {
         
         printStatus.textContent = 'Sending to printer...';
         
-        // Configure QZ Tray for PDF printing with 4x8cm size
+        // Configure QZ Tray for PDF printing with 8x5cm size in landscape
         const config = qzTray.configs.create(selectedPrinter, {
-            size: { width: 4, height: 8, units: 'cm' },
-            margins: { top: 0, right: 0, bottom: 0, left: 0 },
-            orientation: 'portrait'
+            scaleContent: false,
+            rasterize: false,
+            size: { width: 80, height: 50 },
+            units: 'mm',
+            margins: { top: 0, right: 0, bottom: 0, left: 0 }
         });
         
         const data = [{