refactor(i18n): ask the catalogue for a sentence in one helper

`t()` answers a missing key with the key, so "does this code have a
sentence" was a comparison written inline at every site that wanted a
code's sentence or a fallback, twenty-odd times in several spellings.
translateOrNull, translateCodeOrNull and translateCode in
src/lib/i18n/translate-code.ts now hold that probe, every site uses
them, and a guard refuses the inline comparison anywhere else.

src/lib/api/localized-error.ts

@@ -1,4 +1,5 @@
 import { ApiError } from "./api-fetch";
+import { translateCode } from "@/lib/i18n/translate-code";
 
 type Translate = (key: string) => string;
 
@@ -12,14 +13,9 @@ export function localizedApiError(
   t: Translate,
   fallbackKey: string,
 ): string {
-  if (error instanceof ApiError) {
-    const errorCode = error.meta?.errorCode;
-    if (typeof errorCode === "string") {
-      const key = `apiErrors.${errorCode}`;
-      const translated = t(key);
-      if (translated !== key) return translated;
-    }
-  }
-
-  return t(fallbackKey);
+  const errorCode =
+    error instanceof ApiError && typeof error.meta?.errorCode === "string"
+      ? error.meta.errorCode
+      : null;
+  return translateCode(t, "apiErrors", errorCode, fallbackKey);
 }