Allow text customization in the embedded customer portal

Allow merchants to customize text labels and strings (e.g., greeting message, section titles) in the embedded customer portal (Shopify Customer Account Extension). Current behavior: The embedded portal displays hardcoded/auto-translated text that cannot be edited by the merchant. For example, "Bon retour" (a poor French translation of "Welcome back") is shown and cannot be changed. Use case: French-speaking merchant needs to fix incorrect auto-translations in the embedded portal. "Bon retour" translates to "Goodbye" in French context, which is unprofessional for their subscribers. They want to change it to "Bonjour".