[IMP] website_sale_aplicoop: improve i18n label fallbacks and response parsing

Add hardcoded fallback translations for es/eu when PO-based translation
returns the source string unchanged. Expand labels dict with all keys
needed by the frontend. Fix JSON response parsing in template
(data.result || data). Add js_translations keys. Add pot file for
stock_picking_batch_custom.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

website_sale_aplicoop/models/js_translations.py

@@ -35,6 +35,8 @@ def _register_translations():
     _("Remove Item")
     _("Add to Cart")
     _("Save as Draft")
+    _("Save Draft")
+    _("Save order as draft")
     _("Load Draft")
     _("Browse Product Categories")