fix(ntfy): improve admin ntfy UX and add clear token button

- Add missing admin.ntfy.hint translation key in all 15 languages
- Add admin ntfy server hint clarifying it is the default for users
- Expose admin_ntfy_server via PreferencesMatrix so user settings
  placeholder reflects the admin-configured default
- Add clear token button to admin ntfy panel (same pattern as user settings)
- Extract common.clear from settings.ntfyUrl.clearToken across all 15 languages

server/src/services/notificationPreferencesService.ts

@@ -89,6 +89,7 @@ export interface PreferencesMatrix {
   available_channels: AvailableChannels;
   event_types: NotifEventType[];
   implemented_combos: Record<NotifEventType, NotifChannel[]>;
+  defaults?: { ntfyServer: string | null };
 }
 
 /**
@@ -152,6 +153,7 @@ export function getPreferencesMatrix(userId: number, userRole: string, scope: 'u
     available_channels,
     event_types,
     implemented_combos: IMPLEMENTED_COMBOS,
+    ...(scope === 'user' && { defaults: { ntfyServer: getAppSetting('admin_ntfy_server') || null } }),
   };
 }