fix: add the saas onboarding pattern to the shop, checkout and order received

[YvesCesar]

Closes #19.

The libresign/saas-onboarding pattern was referenced but never registered, so nothing was rendered on the shop, checkout and order received pages.

• Adds patterns/saas-onboarding.php: a heading and three steps (choose a plan, create your workspace, start signing). Strings are in English under the libresign text domain, so they reach Weblate through the usual .pot update.
• Calls the pattern from the archive-product and page-checkout templates, as order-confirmation already did.
• Removes the prepend from the the_content filter in inc/account.php. On the shop it never ran, since archive-product has no post-content block, and on the order received page the template already rendered the pattern. The function now only handles the account page, so it is renamed to libresign_theme_render_account_content.
• Hides the onboarding when paying an existing order (order-pay) and when the received order is a renewal, switch or resubscribe of an existing subscription (inc/onboarding.php), since that customer already has a workspace.
• Loads WooCommerce Subscriptions in the integration harness (same package as woocommerce-restrict-switch) to cover those orders.
• Integration test renders the three templates and checks the onboarding appears once in each.
• E2E test checks the onboarding on the real shop, checkout and order received pages, and its absence on a product page.

After deploy, the archive-product template saved in the database through the Site Editor overrides the theme file. Clearing its customizations (as listed in #13) is needed for the shop to show the onboarding.