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/pages/DataQuality.tsx

@@ -1,11 +1,12 @@
 import { useCallback, useEffect, useState } from "react";
 import { Link } from "react-router-dom";
 import { useTranslation } from "react-i18next";
-import { api, ApiError } from "../api/client";
+import { api } from "../api/client";
+import { describeApiError, type ApiErrorInfo } from "../api/errorMessages";
 import type { DataQualityIssue, ScanResult } from "../api/types";
 import { useAuth } from "../context/AuthContext";
 import { SeverityBadge, StatusBadge } from "../components/Badge";
-import { EmptyState, ErrorState, LoadingState, PageHeader } from "../components/PageChrome";
+import { ApiErrorNotice, EmptyState, ErrorState, LoadingState, PageHeader } from "../components/PageChrome";
 
 const RULE_TYPES = [
   "possible_duplicate_customer",
@@ -23,7 +24,7 @@ export function DataQuality() {
   const [status, setStatus] = useState("open");
   const [ruleType, setRuleType] = useState("");
   const [scanning, setScanning] = useState(false);
-  const [scanError, setScanError] = useState<string | null>(null);
+  const [scanError, setScanError] = useState<ApiErrorInfo | null>(null);
   const [scanResult, setScanResult] = useState<ScanResult | null>(null);
   const [confirmingScan, setConfirmingScan] = useState(false);
   const [demoScenariosOnly, setDemoScenariosOnly] = useState(false);
@@ -54,7 +55,7 @@ export function DataQuality() {
       setConfirmingScan(false);
       load();
     } catch (err) {
-      setScanError(err instanceof ApiError ? err.message : t("list.scanFailed"));
+      setScanError(describeApiError(t, err, "list.scanFailed"));
     } finally {
       setScanning(false);
     }
@@ -101,7 +102,7 @@ export function DataQuality() {
         }
       />
 
-      {scanError && <p className="error" role="alert">{scanError}</p>}
+      <ApiErrorNotice error={scanError} />
       {scanResult && (
         <p className="quiet-empty" role="status">
           {t("list.scanComplete", {