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/App.jsx

@@ -9,6 +9,7 @@ import Dashboard from "./pages/Dashboard.jsx";
 import RegistrationPage from './pages/RegistrationPage.jsx';
 import SettingsPage from './pages/SettingsPage.jsx';
 import AdminHintsPage from './pages/AdminHintsPage.jsx';
+import AdminBroadcastPage from './pages/AdminBroadcastPage.jsx';
 import AdminNewsPage from './pages/AdminNewsPage.jsx';
 import WhatsNewPage from './pages/WhatsNewPage.jsx';
 import WelcomePage from './pages/WelcomePage.jsx';
@@ -82,6 +83,7 @@ function App() {
                             <Route path="/admin/users" element={<AdminUsersPage/>}/>
                             <Route path="/admin/hints" element={<AdminHintsPage/>}/>
                             <Route path="/admin/news" element={<AdminNewsPage/>}/>
+                            <Route path="/admin/broadcasts" element={<AdminBroadcastPage/>}/>
                         </Route>
                         <Route path="/u/:name" element={<PublicProfilePage/>}/>
                         <Route path="/impressum" element={<ImprintPage/>}/>