Added a new feature that allows viewing all product barcodes for a tenant
in a grid layout with print-optimized styling. This makes it easy to print
barcode sheets for all products at once.
Changes:
- Add new route /<tenant_id>/barcodes for viewing all barcodes
- Create view_all_barcodes() function in routes/admin.py
- Add all_barcodes.html template with responsive grid and print CSS
- Add "View All Barcodes" button to tenant index page
- Print layout optimized for 3-column barcode sheets
