fix: icones de notificacao e splash ainda usavam o artwork do FluffyChat

- notifications_icon.png: gera a silhueta branca a partir do logo atual
  nas 5 densidades (era o drawable do Initial commit, nunca trocado)
- splash.png: idem, mantendo os canvas originais e o logo a 64% da largura
- apaga ic_launcher_foreground.xml e ic_launcher_monochrome.xml, vectors
  antigos com o roxo do FluffyChat que competiam com os PNGs por densidade
- foreground service passa a usar notifications_icon em vez do inexistente
  ic_launcher (caia no icone do launcher)
- bumpara o id do canal para tatuchat_push_v2, senao o Android mantem o
  icone antigo congelado no canal ja criado
- titulo do foreground service: FluffyChat -> TatuChat

lib/utils/start_push_foreground_service.dart

@@ -68,9 +68,11 @@ abstract class ForegroundServices {
         }
         final result = await FlutterForegroundTask.startService(
           serviceTypes: [ForegroundServiceTypes.shortService],
-          notificationTitle: 'FluffyChat',
+          notificationTitle: 'TatuChat',
           notificationText: l10n.loadingMessages,
-          notificationIcon: NotificationIcon(metaDataName: 'ic_launcher'),
+          notificationIcon: NotificationIcon(
+            metaDataName: 'notifications_icon',
+          ),
         );
         final started = result is ServiceRequestSuccess;
         Logs().d('[PushHelper] Foreground service start: $started ($result)');