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/PreferencesCard.jsx

@@ -11,7 +11,7 @@ import {usePreferences} from '../context/PreferencesContext.jsx';
 import usePersistSettings from '../hooks/usePersistSettings.js';
 import useSaveResponse from '../hooks/useSaveResponse.js';
 import {FAVORITE_PLATFORMS, saveFavoritePlatform} from '../utils/platforms.js';
-import {LANGUAGES, saveSettings} from '../utils/settings.js';
+import {LANGUAGES, savePlatformMails, saveSettings} from '../utils/settings.js';
 
 // Radix Select has no empty value.
 const NO_FAVORITE = 'NONE';
@@ -41,6 +41,13 @@ export default function PreferencesCard() {
         }
     };
 
+    const handlePlatformMailsChange = async (next) => {
+        if (await runSave(() => savePlatformMails(next))) {
+            updateProfile((p) => ({...p, settings: {...p.settings, platformMails: next}}));
+            toast.success(t('SETTINGS.SAVED'));
+        }
+    };
+
     return (
         <Card>
             <CardHeader>
@@ -83,6 +90,14 @@ export default function PreferencesCard() {
                         </SelectContent>
                     </Select>
                 </div>
+                <div className="flex items-center justify-between gap-4">
+                    <div className="space-y-1">
+                        <Label htmlFor="platformMails">{t('SETTINGS.PLATFORM_MAILS')}</Label>
+                        <p className="text-xs text-muted-foreground">{t('SETTINGS.PLATFORM_MAILS_HINT')}</p>
+                    </div>
+                    <Switch id="platformMails" checked={profile?.settings?.platformMails ?? true}
+                            onCheckedChange={handlePlatformMailsChange}/>
+                </div>
             </CardContent>
         </Card>
     );