fix(promocode): correct activation UX, error mapping and validity dates

Map promocode errors by stable backend code (detail.code) instead of brittle English
substring matching, so previously-unmapped codes (active_discount_exists, daily_limit,
no_subscription_for_days, trial_*, etc.) show the right message instead of 'server error';
added the missing keys to all 4 locales.

valid_until now anchors to end-of-day in the admin's LOCAL timezone on both create AND
edit read-back, so codes no longer expire at the start of the day and editing no longer
drifts the date forward for negative-offset admins.

Allow 0-hour discount ('until first purchase'). Route user-facing discount deactivation
to the endpoint that rolls back promocode usage when the discount came from a promocode,
and parse the structured {code,message} error.

src/locales/en.json

@@ -787,6 +787,13 @@
         "not_yet_valid": "Promo code is not yet active",
         "used": "Promo code has already been used",
         "already_used_by_user": "You have already used this promo code",
+        "active_discount_exists": "You already have an active discount. Deactivate it first.",
+        "no_subscription_for_days": "This promo code requires an active or expired subscription",
+        "subscription_not_found": "Subscription not found",
+        "not_first_purchase": "This promo code is only available for your first purchase",
+        "daily_limit": "Too many promo code activations today",
+        "trial_subscription_exists": "You already have a subscription, so this trial code can't be applied",
+        "trial_provisioning_failed": "Couldn't provision the trial right now, please try again later",
         "user_not_found": "User not found",
         "server_error": "Server error"
       },