feat(broadcasts): add admin broadcast page and user opt-out switch

Admin page /admin/broadcasts to write, test and send broadcasts in both
languages; confirms the recipient count before send. History shows the
counts. A new switch in settings lets users opt out of platform mails;
account mails always come. Subject and body are plain text only.

src/utils/broadcasts.js

@@ -0,0 +1,30 @@
+import {apiFetch} from './api.js';
+
+const AUTH = () => import.meta.env.VITE_API_URL;
+const JSON_HEADERS = {'Content-Type': 'application/json'};
+
+/**
+ * PLATFORM_ADMIN only (403 otherwise): {results: [{id, subjectEn, subjectDe, bodyEn, bodyDe, createdBy, createdAt,
+ * recipientCount, sentCount, failedCount}]}, newest first.
+ */
+export const fetchBroadcasts = () => apiFetch(`${AUTH()}/api/admin/broadcasts`);
+
+/**
+ * {count}: the active users with platform mails on, among `userIds`; all of them when `userIds` is null.
+ */
+export const fetchRecipientCount = (userIds) =>
+    apiFetch(`${AUTH()}/api/admin/broadcasts/recipient-count${userIds ? `?userIds=${userIds.join(',')}` : ''}`);
+
+/**
+ * `broadcast` is {subjectEn, subjectDe, bodyEn, bodyDe, userIds}. 201 {recipientCount}, 409 NO_RECIPIENTS, or 400 for a
+ * blank or too long field.
+ */
+export const sendBroadcast = (broadcast) =>
+    apiFetch(`${AUTH()}/api/admin/broadcasts`, {method: 'POST', headers: JSON_HEADERS, body: JSON.stringify(broadcast)});
+
+/**
+ * 204: the draft goes to the admin's own address only, in the admin's language.
+ */
+export const sendTestBroadcast = (broadcast) =>
+    apiFetch(`${AUTH()}/api/admin/broadcasts/test`,
+        {method: 'POST', headers: JSON_HEADERS, body: JSON.stringify(broadcast)});

src/utils/limits.js

@@ -33,3 +33,6 @@ export const MAX_HINT_LENGTH = 500;
 // Auth NewsRequest @Size(max = 100) and @Size(max = 2000)
 export const MAX_NEWS_TITLE_LENGTH = 100;
 export const MAX_NEWS_TEXT_LENGTH = 2000;
+// Auth BroadcastRequest @Size(max = 150) and @Size(max = 5000)
+export const MAX_BROADCAST_SUBJECT_LENGTH = 150;
+export const MAX_BROADCAST_BODY_LENGTH = 5000;

src/utils/settings.js

@@ -58,3 +58,11 @@ export const markWelcomeSeen = () =>
         method: 'PUT',
         signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
     });
+
+/** PUT on Auth, 204: turns the broadcast mails on or off. The caller handles the response status. */
+export const savePlatformMails = (enabled) =>
+    apiFetch(`${import.meta.env.VITE_API_URL}/api/users/settings/platform-mails`, {
+        method: 'PUT',
+        headers: {'Content-Type': 'application/json'},
+        body: JSON.stringify({enabled}),
+    });