feat(checkout): show a German message when a checkout step fails

checkoutErrorMessage (utils/checkoutError.ts) maps the status and body of a
failed request, the shop's answer or a raw CMS (Strapi) error, to one fixed
message for the customer, addressed with "du": an invalid e-mail address
(when data.email is among the rejected fields), other invalid input, an order
already paid, a product no longer available, an order not ready for checkout
(naming the missing steps), a payment that does not fit the order or an order
changed during the payment ("Bitte starte die Zahlung neu"), PayPal not
reachable ("versuche es in ein paar Minuten noch einmal"), a payment PayPal
may have taken that the CMS could not confirm or record ("Bitte bezahle nicht
noch einmal"), and a general fallback. It never shows the server's text.

Steps 1 and 2 show the message above their submit button, in the style of
step 3's payment error, instead of logging the error only. Step 3 shows it
for a failed PayPal order creation or capture. The e-mail input of step 1 is
type="email" (Input.vue takes a type).

Refs libretech/mp#71

components/Input.vue

@@ -5,6 +5,7 @@
 		<span :class="spanClasses" class="absolute left-2 top-0 transition-all select-none">{{ label }}</span>
 		<input
 			:value="modelValue"
+			:type="type"
 			@input="update(($event.target as HTMLInputElement).value)"
 			:required="required"
 			class="outline-none w-full h-full rounded-sm pt-4 p-2 border-transparent"
@@ -40,9 +41,11 @@ const props = withDefaults(
 		labelClass?: string;
 		inputClass?: string;
 		autocomplete?: Autocomplete;
+		type?: "text" | "email";
 	}>(),
 	{
-		autocomplete: "off"
+		autocomplete: "off",
+		type: "text"
 	}
 );