fix(i18n): persist User.locale so background messages match the UI language

The language switcher wrote the choice to localStorage, the
`healthlog-locale` cookie and `<html lang>` only — never to the user row.
Server-side background work (the proactive Coach nudge, the Telegram test
message) has no request cookie to read and falls back to `User.locale`;
with the column left null those messages went out in English regardless of
the language the user reads the app in.

Add `PUT /api/auth/me/locale` (idempotent, validated against the supported
locales) and have the i18n provider mirror the active locale onto the row —
on every switch and once on mount, so the column is backfilled for users
who set their language before this landed. Fire-and-forget: a 401 on a
public page or an offline blip never blocks the UI flip; the cookie stays
the first-paint source of truth.

src/app/api/auth/me/locale/route.ts

@@ -0,0 +1,62 @@
+/**
+ * PUT /api/auth/me/locale
+ *
+ * v1.25.0 — persist the user's UI locale onto `User.locale`.
+ *
+ * The language switcher historically wrote the choice to localStorage,
+ * the `healthlog-locale` cookie and `<html lang>` ONLY — never to the
+ * user row. Server-side background work (the proactive Coach nudge, the
+ * Telegram test message, …) has no request cookie to read, so it falls
+ * back to `User.locale`; with the column left null those messages went
+ * out in English regardless of the language the user reads the app in.
+ *
+ * This setter closes the gap: the client persists the active locale here
+ * (on every switch, plus a one-time backfill on mount) so the column
+ * reliably mirrors the UI language and the cron paths render in it.
+ *
+ * Auth: cookie session OR Bearer token.
+ * Validation: `{ "locale": "de" }` — must be one of the supported
+ * locales; anything else is rejected at the surface so the column never
+ * holds a value the resolver would later have to defend against.
+ */
+import { NextRequest } from "next/server";
+import { prisma } from "@/lib/db";
+import { apiHandler, requireAuth } from "@/lib/api-handler";
+import { apiError, apiSuccess, safeJson } from "@/lib/api-response";
+import { annotate } from "@/lib/logging/context";
+import { locales, type Locale } from "@/lib/i18n/config";
+
+export const dynamic = "force-dynamic";
+
+function isLocale(value: unknown): value is Locale {
+  return (
+    typeof value === "string" && (locales as readonly string[]).includes(value)
+  );
+}
+
+export const PUT = apiHandler(async (request: NextRequest) => {
+  const { user } = await requireAuth();
+  annotate({ action: { name: "user.locale.update" } });
+
+  const { data: body, error: jsonError } = await safeJson(request, {
+    maxBytes: 4 * 1024,
+  });
+  if (jsonError) return jsonError;
+
+  const locale = (body as { locale?: unknown } | null)?.locale;
+  if (!isLocale(locale)) {
+    return apiError("Not a supported locale.", 422);
+  }
+
+  // Idempotent: skip the write (and the annotation noise) when the column
+  // already matches — the mount-time backfill fires on every page load.
+  if (user.locale !== locale) {
+    await prisma.user.update({
+      where: { id: user.id },
+      data: { locale },
+    });
+    annotate({ meta: { locale_next: locale } });
+  }
+
+  return apiSuccess({ locale });
+});

src/lib/i18n/context.tsx

@@ -89,6 +89,26 @@ function persistLocale(newLocale: Locale) {
   document.documentElement.lang = newLocale;
 }
 
+// v1.25.0 — mirror the active locale onto `User.locale` so server-side
+// background work (the proactive Coach nudge, the Telegram test message)
+// renders in the user's language. The cookie/localStorage choice is
+// invisible to a cron that has no request context; without this write the
+// column stays null and those messages fall back to English. Fire-and-forget
+// and idempotent on the server: a 401 on a public page, an offline blip or a
+// no-op equal value all fail silently and never block the UI flip.
+function persistLocaleToServer(newLocale: Locale) {
+  void fetch("/api/auth/me/locale", {
+    method: "PUT",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({ locale: newLocale }),
+    credentials: "same-origin",
+    keepalive: true,
+  }).catch(() => {
+    // Best-effort: the cookie remains the first-paint source of truth, and
+    // the next mount retries the backfill.
+  });
+}
+
 export function I18nProvider({
   children,
   initialLocale,
@@ -152,6 +172,14 @@ export function I18nProvider({
     };
   }, [locale]);
 
+  // v1.25.0 — keep `User.locale` in step with the committed UI locale. Runs on
+  // mount (backfilling the column for users who set their language before this
+  // landed) and on every switch, so the proactive Coach nudge and other
+  // cookie-blind background paths render in the language the user reads.
+  useEffect(() => {
+    persistLocaleToServer(locale);
+  }, [locale]);
+
   const setLocale = useCallback((newLocale: Locale) => {
     if (!(locales as readonly string[]).includes(newLocale)) return;