NTR: improve checkout payment form accessibility

[cxo-jutz]

Brings the Mollie phone and credit card payment fields in line with our form conventions (native HTML5 semantics as the first layer, ARIA for state and announcements). Storefront templates + phone plugin only — no logic or API changes.

Phone field (phone-fields.html.twig)

• type="text" → type="tel" + inputmode="tel" (correct mobile keyboard) and autocomplete="tel" (was off, blocked autofill). The submitted value stays a plain string — type="tel" does no numeric coercion.
• Removed the non-standard invalid="true" attribute. The error element now has an id, role="alert" (live announcement when shown), and is linked to the input via aria-errormessage, which is exposed once aria-invalid="true" is set.
• pattern (E.164-style) kept as the HTML5 validation layer.

Phone plugin (phone-plugin.js)

• Toggles the standard aria-invalid attribute (string value) instead of the invented invalid attribute — this is what activates aria-errormessage and the [aria-invalid] styling hook.

Credit card fields (cc-fields.html.twig)

• The four <label for="…"> pointed at <div> iframe mount containers, which is ignored by assistive tech. Labels now carry an id, and each mount container references it via aria-labelledby and is exposed as role="group".
• The Error containers (populated by Mollie Components) are now role="alert" so component-level errors are announced.