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/pages/SettingsPage.test.jsx

@@ -31,6 +31,7 @@ const authRoutes = (me) => ({
     'PUT /api/users/settings/appearance': noBody,
     'PUT /api/users/settings/profile-style': noBody,
     'PUT /api/users/settings/favorite-platform': noBody,
+    'PUT /api/users/settings/platform-mails': noBody,
     'GET /api/version': json(404, {}),
 });
 
@@ -247,6 +248,29 @@ describe('SettingsPage favorite platform', () => {
     });
 });
 
+describe('SettingsPage platform mails', () => {
+    beforeEach(() => localStorage.setItem('tbp_session', '1'));
+
+    afterEach(() => {
+        cleanup();
+        localStorage.clear();
+    });
+
+    it('is on by default and saves the opt-out', async () => {
+        toast.success.mockClear();
+        renderPage(profile());
+
+        const mails = await screen.findByRole('switch', {name: 'SETTINGS.PLATFORM_MAILS'});
+        expect(mails.getAttribute('aria-checked')).toBe('true');
+        fireEvent.click(mails);
+
+        await waitFor(() => expect(puts()).toEqual([{enabled: false}]));
+        expect(fetch.mock.calls.find(([, init]) => init?.method === 'PUT')[0]).toContain('/api/users/settings/platform-mails');
+        await waitFor(() => expect(toast.success).toHaveBeenCalledWith('SETTINGS.SAVED'));
+        expect(mails.getAttribute('aria-checked')).toBe('false');
+    });
+});
+
 describe('SettingsPage session', () => {
     beforeEach(() => localStorage.setItem('tbp_session', '1'));