[FIX] loyalty, pos_loyalty: download, print, mail gift card

- Gift cards can now be sent alongside the receipt via mail in the Receipt Screen.
- The gift card report no longer show n² gift cards
- Properly download all PDF files for the gift cards when there are multiple gift card programs

closes odoo/odoo#102676

Signed-off-by: Masereel Pierre <pim@odoo.com>