fix(goals): validate color format + restore cascade on drop migration

- Add hex-format validation on Goal#color so submissions can't smuggle
  arbitrary CSS into the style attribute on the avatar / picker preview.
  The picker accepts custom hexes, so format validation (not inclusion)
  is the right shape — anything not matching #RRGGBB is rejected at
  the model boundary.
- Fix the on_delete in the down block of drop_goal_contributions to
  match the original cascade. Restoring with restrict was a schema
  drift that would have shifted referential behavior after a rollback.

app/models/goal.rb

@@ -5,6 +5,7 @@ class Goal < ApplicationRecord
   ICONS = Category.icon_codes
 
   validates :icon, inclusion: { in: ICONS, allow_nil: true }
+  validates :color, format: { with: /\A#[0-9A-Fa-f]{6}\z/ }, allow_nil: true
 
   belongs_to :family
   has_many :goal_accounts, dependent: :destroy

db/migrate/20260514120001_drop_goal_contributions.rb

@@ -6,7 +6,7 @@ class DropGoalContributions < ActiveRecord::Migration[7.2]
   def down
     create_table :goal_contributions, id: :uuid do |t|
       t.references :goal, null: false, foreign_key: { on_delete: :cascade }, type: :uuid
-      t.references :account, null: false, foreign_key: { on_delete: :restrict }, type: :uuid
+      t.references :account, null: false, foreign_key: { on_delete: :cascade }, type: :uuid
       t.decimal :amount, precision: 19, scale: 4, null: false
       t.string :currency, null: false
       t.string :source, default: "manual", null: false