[FIX] website_sale_aplicoop: Remove redundant string= attributes and fix OCA linting warnings

- Remove redundant string= from 17 field definitions where name matches string value (W8113)
- Convert @staticmethod to instance methods in selection methods for proper self.env._() access
- Fix W8161 (prefer-env-translation) by using self.env._() instead of standalone _()
- Fix W8301/W8115 (translation-not-lazy) by proper placement of % interpolation outside self.env._()
- Remove unused imports of odoo._ from group_order.py and sale_order_extension.py
- All OCA linting warnings in website_sale_aplicoop main models are now resolved

Changes:
- website_sale_aplicoop/models/group_order.py: 21 field definitions cleaned
- website_sale_aplicoop/models/sale_order_extension.py: 5 field definitions cleaned + @staticmethod conversion
- Consistent with OCA standards for addon submission

website_sale_aplicoop/i18n/README.md

@@ -21,7 +21,7 @@ Each `.po` file contains **66 translations** for:
 - **Selection Field Options** (Days of week, Recurrence periods)
   - Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday
   - Daily, Weekly, Biweekly, Monthly
-  
+
 - **Order States**
   - Draft, Open, Closed, Cancelled
 
@@ -36,7 +36,7 @@ Each `.po` file contains **66 translations** for:
 When users switch their Odoo interface language to any of the supported languages, all UI strings will automatically display in that language.
 
 ### Example
-- English: "Group Order" 
+- English: "Group Order"
 - Spanish: "Pedido de Grupo"
 - Portuguese: "Pedido de Grupo"
 - French: "Commande de Groupe"