Updates the header to use Shopify's customer account experience, adds a Customer accounts theme setting, and removes the legacy customer account pages.

### Removed

- Important: [Customer accounts] Removed the legacy customer account templates and sections. Stores on legacy customer accounts must upgrade to new customer accounts before publishing this version. See information on [upgrading to customer accounts from legacy customer accounts](https://help.shopify.com/manual/customers/customer-accounts/upgrade)

### Added

- [Customer accounts] New Customer accounts theme setting with a Menu setting for the customer account popover.

### Changed

- [Customer accounts] The header account icon now opens Shopify's customer account popover instead of a separate login page.

### Fixes and improvements

- [Mobile menu] Country/region and language selectors now respect their Header section settings, matching the desktop header.
- [Mobile menu] Fixed footer spacing when social links aren't shown.