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/ios/Runner/Info.plist

@@ -4,6 +4,8 @@
 <dict>
 	<key>CADisableMinimumFrameDurationOnPhone</key>
 	<true/>
+	<key>NSUserNotificationsUsageDescription</key>
+	<string>Halo Bestie kirim notifikasi pas bestie udah siap dengerin dan pas ada chat baru.</string>
 	<key>CFBundleDevelopmentRegion</key>
 	<string>$(DEVELOPMENT_LANGUAGE)</string>
 	<key>CFBundleDisplayName</key>