Fix ArgumentError for text column length validations

`type_for_attribute(:description).limit` returns nil for text columns,
which newer Rails rejects with ArgumentError: :maximum must be a
non-negative Integer. Guard with `if type_for_attribute(...).limit`
to skip the validation when the column has no limit.

Affects Unit and Quantity models.

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}
+  validates :description, length: {maximum: type_for_attribute(:description).limit} if type_for_attribute(:description).limit
 
   # Update :depths of progenies after parent change
   before_save if: :parent_changed? do