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>

app/models/quantity.rb

@@ -16,7 +16,7 @@ class Quantity < ApplicationRecord
   end
   validates :name, presence: true, uniqueness: {scope: [:user_id, :parent_id]},
     length: {maximum: type_for_attribute(:name).limit}
-  validates :description, length: {maximum: type_for_attribute(:description).limit} if type_for_attribute(:description).limit
+  validates :description, length: {maximum: type_for_attribute(:description).limit}
 
   # Update :depths of progenies after parent change
   before_save if: :parent_changed? do

app/models/unit.rb

@@ -13,7 +13,7 @@ class Unit < ApplicationRecord
   end
   validates :symbol, presence: true, uniqueness: {scope: :user_id},
     length: {maximum: type_for_attribute(:symbol).limit}
-  validates :description, length: {maximum: type_for_attribute(:description).limit} if type_for_attribute(:description).limit
+  validates :description, length: {maximum: type_for_attribute(:description).limit}
   validates :multiplier, numericality: {equal_to: 1}, unless: :base
   validates :multiplier, numericality: {greater_than: 0, precision: true, scale: true}, if: :base
 

db/seeds.rb

@@ -21,3 +21,4 @@ end
 #[Source, Quantity, Unit].each { |model| model.defaults.delete_all }
 
 require_relative 'seeds/units.rb'
+require_relative 'seeds/quantities.rb'

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