Phase 4 Stage 4: notif gate + home permission-denied banner

Notif Gate full screen at /onboarding/notif-gate, reached from waiting
payment on confirmed/consumed status. Auto-advances to /chat/searching
when permission is already granted; otherwise shows izinkan/nanti aja
HaloButton CTAs. NotifPermission helper wraps firebase_messaging +
permission_handler with readStatus/request/openAppSettings; cached in
notifPermissionStatusProvider that re-reads on app foreground via an
internal WidgetsBindingObserver.

home_screen amber banner above-the-fold when notifPermissionStatusProvider
reports denied. Dismissable for the session via homeNotifBannerDismissedProvider
(in-memory StateProvider, no persistence - cold-restart re-shows).
nyalain CTA -> openAppSettings().

Manifest + Info.plist permission entries added.

Note: main.dart still pre-requests FirebaseMessaging permission at boot,
which can pre-resolve status so the gate auto-advances instead of acting
as the first prompt. Left intact for now; can be removed in a later
stage if the gate should be the first-ask UX.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/windows/flutter/generated_plugin_registrant.cc

@@ -8,10 +8,13 @@
 
 #include <firebase_core/firebase_core_plugin_c_api.h>
 #include <flutter_secure_storage_windows/flutter_secure_storage_windows_plugin.h>
+#include <permission_handler_windows/permission_handler_windows_plugin.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
   FirebaseCorePluginCApiRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("FirebaseCorePluginCApi"));
   FlutterSecureStorageWindowsPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("FlutterSecureStorageWindowsPlugin"));
+  PermissionHandlerWindowsPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("PermissionHandlerWindowsPlugin"));
 }