fix: populate empty checkout-datetime locale file

extensions/checkout-datetime/locales/en.default.json was an empty {}
(unlike pos-datetime and datetime-widget, which both have real content) —
shopify app dev's checkout preview refuses to start with "Locale data for
`en` is empty." Checkout.jsx/ThankYou.jsx don't call useTranslate yet (all
strings are hardcoded inline), so this just satisfies the non-empty
requirement and mirrors the extension's visible strings for when it is
wired up to i18n.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

extensions/checkout-datetime/locales/en.default.json

@@ -1 +1,20 @@
-{}
+{
+  "name": "Delivery Date & Time",
+  "description": "Native pickup/delivery date & time picker in checkout, plus a Thank You page confirmation.",
+  "checkout": {
+    "heading": "Choose your delivery date",
+    "method_label": "Method",
+    "method_pickup": "Pickup",
+    "method_local_delivery": "Local delivery",
+    "method_shipping": "Shipping",
+    "date_label": "Date",
+    "time_label": "Time",
+    "loading": "Loading…",
+    "no_dates": "No dates available.",
+    "confirmed_heading": "Delivery date & time",
+    "confirmed_prefix": "Confirmed for",
+    "error_load": "Couldn't load available dates.",
+    "error_hold": "That slot was just taken.",
+    "error_confirm": "Couldn't reserve that slot."
+  }
+}