feat: centralize API error localization

Replace the err instanceof ApiError ? err.message : t(fallback) anti-
pattern -- which showed raw English backend text for the common case and
only used the localized fallback for the rare network-failure case -- at
all 13 call sites across 7 files.

New frontend/src/api/errorMessages.ts (describeApiError) resolves a
caught error to a localized {title, explanation, nextStep?, technical}
by checking the 32 known AppError codes first, then known HTTP statuses
(401/403/404/409/422/500), then a fully generic fallback. New
ApiErrorNotice (PageChrome.tsx) renders title/explanation/nextStep with
the raw text demoted to a "Technical details"/"Details techniques"
disclosure -- never shown as the primary message.

ApiError itself is split out of client.ts into a standalone
api/apiError.ts with no import.meta.env dependency, so errorMessages.ts
(and its tests) can be loaded outside a Vite/browser context.

frontend/src/components/PageChrome.tsx

@@ -67,6 +67,26 @@ export function ErrorState({ message }: { message: string }) {
   );
 }
 
+// Shared rendering for describeApiError()'s output: a localized title + explanation +
+// optional next step, with the raw backend/network text demoted to a "Technical
+// details" disclosure -- never shown as the primary message. See
+// frontend/src/api/errorMessages.ts and docs/fleet-ops-final-localization/audit.md.
+export function ApiErrorNotice({ error }: { error: import("../api/errorMessages").ApiErrorInfo | null }) {
+  const { t } = useTranslation("common");
+  if (!error) return null;
+  return (
+    <div className="error api-error-notice" role="alert">
+      <strong>{error.title}</strong>
+      <p>{error.explanation}</p>
+      {error.nextStep && <p className="api-error-next-step">{error.nextStep}</p>}
+      <details className="evidence-disclosure">
+        <summary>{t("actions.technicalDetails")}</summary>
+        <pre className="evidence-block">{error.technical}</pre>
+      </details>
+    </div>
+  );
+}
+
 export function EmptyState({
   icon = "check",
   title,