docs(api): document apiClient error.status / errorCode / details surface

Adds a "Frontend Error Handling (apiClient)" section to the error
handling guide. Callers can branch on `error.errorCode` and read
`error.details` to localise toasts instead of rendering the raw
English `message`, as already done in the loyalty terminal cooldown
handler.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/api/error-handling.md

@@ -284,6 +284,33 @@ async def invalid_token_handler(request, exc):
     )
 ```
 
+## Frontend Error Handling (apiClient)
+
+When a request fails, the shared `apiClient` (`static/shared/js/api-client.js`) throws an `Error` with three structured properties populated from the JSON response, in addition to the `message`:
+
+| Property | Source | Example |
+|----------|--------|---------|
+| `error.status` | HTTP status code | `429` |
+| `error.errorCode` | Server `error_code` field | `"POINTS_COOLDOWN"` |
+| `error.details` | Server `details` object (may be `null`) | `{ cooldown_minutes: 5, cooldown_ends: "..." }` |
+
+This lets callers branch on a machine-readable code and localise the toast instead of rendering the raw English `message`. Pattern:
+
+```js
+try {
+    await apiClient.post('/store/loyalty/transactions', payload);
+} catch (error) {
+    if (error.errorCode === 'POINTS_COOLDOWN' || error.errorCode === 'STAMP_COOLDOWN') {
+        const minutes = error.details?.cooldown_minutes ?? '';
+        Utils.showToast(I18n.t('loyalty.store.terminal.cooldown_wait_minutes', { minutes }), 'warning');
+    } else {
+        Utils.showToast(I18n.t('loyalty.store.terminal.transaction_failed', { message: error.message }), 'error');
+    }
+}
+```
+
+Generic FastAPI errors that don't carry `error_code` / `details` will leave those properties `undefined` / `null`, so a `?.` chain is the right guard.
+
 ## Related Documentation
 
 - [Authentication](authentication.md) - Authentication-related exceptions