Use browser Accept-Language for login and onboarding locale (#768)

* Use Accept-Language for unauthenticated locale

* Add per-user locale overrides

* Fix test

* Use more than the top `accept-language` entry

* Localization of string

db/migrate/20260124180211_add_locale_to_users.rb

@@ -0,0 +1,6 @@
+class AddLocaleToUsers < ActiveRecord::Migration[7.2]
+  def change
+    add_column :users, :locale, :string, null: true, default: nil
+    add_index :users, :locale
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2026_01_23_214127) do
+ActiveRecord::Schema[7.2].define(version: 2026_01_24_180211) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -1402,9 +1402,11 @@ ActiveRecord::Schema[7.2].define(version: 2026_01_23_214127) do
     t.datetime "set_onboarding_goals_at"
     t.string "default_account_order", default: "name_asc"
     t.jsonb "preferences", default: {}, null: false
+    t.string "locale"
     t.index ["email"], name: "index_users_on_email", unique: true
     t.index ["family_id"], name: "index_users_on_family_id"
     t.index ["last_viewed_chat_id"], name: "index_users_on_last_viewed_chat_id"
+    t.index ["locale"], name: "index_users_on_locale"
     t.index ["otp_secret"], name: "index_users_on_otp_secret", unique: true, where: "(otp_secret IS NOT NULL)"
     t.index ["preferences"], name: "index_users_on_preferences", using: :gin
   end