Add default Body weight quantity to seeds

Introduces db/seeds/quantities.rb with a default "Body weight" quantity
(user_id nil = system default), mirroring the pattern used by seeds/units.rb.
Wires it into db/seeds.rb so it runs on db:seed.

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

db/seeds/quantities.rb

@@ -0,0 +1,9 @@
+Quantity.transaction do
+  Quantity.defaults.delete_all
+
+  quantities = {}
+
+  quantities['Body weight'] =
+    Quantity.create name: 'Body weight',
+                    description: 'body weight measurement'
+end