Add: Weight field to product admin pages

Added weight (in kilograms) field to both create and edit product pages.
Admins can now set the weight for each product, which is converted to
grams and stored in the database for Royal Mail shipping calculations.

.claude/settings.local.json

@@ -86,7 +86,8 @@
       "Bash(git commit -m 'Feature: Add product selection when starting design from photo request *)",
       "Bash(git commit -m 'Refactor: Move start design server action to separate file *)",
       "Bash(git commit -m 'Add: Prominent '\\\\''Shop plain items'\\\\'' section to homepage *)",
-      "Bash(git commit -m 'Feature: Royal Mail shipping integration with weight-based postage *)"
+      "Bash(git commit -m 'Feature: Royal Mail shipping integration with weight-based postage *)",
+      "Bash(git commit -m 'Add: Weight field to product admin pages *)"
     ]
   }
 }