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/components/AppHeader.jsx

@@ -1,7 +1,7 @@
 import {useEffect, useState} from 'react';
 import {Link, NavLink, useLocation, useNavigate} from 'react-router-dom';
 import {useTranslation} from 'react-i18next';
-import {ChartColumn, Coins, Dices, Eye, Languages, LayoutDashboard, Library, LogOut, Megaphone, Moon, Newspaper, Settings, Shield, Sun} from 'lucide-react';
+import {ChartColumn, Coins, Dices, Eye, Languages, LayoutDashboard, Library, LogOut, Mail, Megaphone, Moon, Newspaper, Settings, Shield, Sun} from 'lucide-react';
 import {toast} from 'sonner';
 
 import {Badge} from '@/components/ui/badge';
@@ -169,6 +169,9 @@ export default function AppHeader({isAuthenticated, onLogout}) {
                                         <DropdownMenuItem asChild>
                                             <Link to="/admin/news"><Newspaper/>{t('NAV.ADMIN_NEWS')}</Link>
                                         </DropdownMenuItem>
+                                        <DropdownMenuItem asChild>
+                                            <Link to="/admin/broadcasts"><Mail/>{t('NAV.ADMIN_BROADCASTS')}</Link>
+                                        </DropdownMenuItem>
                                     </>
                                 )}
                                 {isAdmin && (