Default label for carrier labels

This module gives you a fallback shipping label you can print from a transfer (stock.picking) when no carrier-specific label exists.

• A new PDF report called "Shipping Label" is available on transfers. It is also listed in the Print menu of the transfer form thanks to its report binding.
• The label uses an A7 landscape paper format with small margins, so it fits standard label sheets.
• The printed content is simply the recipient's contact address taken from the transfer's contact (partner_id), rendered with the standard contact widget.
• The report can be generated for several transfers at once (multi) and, when generated, the resulting file is stored as an attachment (attachment_use), so it can be reused instead of being regenerated.
• A technical method on the transfer, generate_default_label, produces the label file in base64 and returns its name and file type; this is what carrier integrations can call to obtain a default label.
• The module depends on stock_delivery and delivery_carrier_shipping_label, so it plugs into the existing delivery and carrier label workflow.

It does not add menus, checks, or automatic emails; it only provides the report, its paper format, and the helper method on transfers.