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/lib/features/onboarding/screens/notif_gate_screen.dart

@@ -0,0 +1,136 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+
+import '../../../core/notifications/notif_permission.dart';
+import '../../../core/theme/halo_tokens.dart';
+import '../../../core/theme/widgets/widgets.dart';
+
+/// Phase 4 Stage 4 — post-payment notif gate.
+///
+/// If the OS notif permission is already granted on entry, the screen
+/// auto-advances to the searching shell. Otherwise it offers two CTAs:
+/// "izinkan notifikasi" (request) and "nanti aja" (skip). Either resolution
+/// path advances to the searching shell — Stage 5 will refine that target if
+/// the post-payment pairing entry needs to differ.
+// TODO(stage5): if Stage 5 introduces a dedicated post-payment pairing entry,
+// retarget the advance from `/chat/searching` to whichever route owns the
+// blast trigger after a paid session.
+class NotifGateScreen extends ConsumerStatefulWidget {
+  const NotifGateScreen({super.key});
+
+  @override
+  ConsumerState<NotifGateScreen> createState() => _NotifGateScreenState();
+}
+
+class _NotifGateScreenState extends ConsumerState<NotifGateScreen> {
+  static const String _advanceRoute = '/chat/searching';
+
+  bool _resolving = false;
+  bool _autoAdvanced = false;
+
+  void _advance() {
+    if (!mounted) return;
+    context.go(_advanceRoute);
+  }
+
+  Future<void> _onAllow() async {
+    if (_resolving) return;
+    setState(() => _resolving = true);
+    await ref.read(notifPermissionStatusProvider.notifier).request();
+    if (!mounted) return;
+    _advance();
+  }
+
+  void _onLater() => _advance();
+
+  @override
+  Widget build(BuildContext context) {
+    final statusAsync = ref.watch(notifPermissionStatusProvider);
+
+    // Already-granted users skip the screen entirely. Schedule the redirect
+    // post-frame so we don't `context.go` mid-build.
+    if (!_autoAdvanced &&
+        statusAsync.valueOrNull == NotifPermStatus.granted) {
+      _autoAdvanced = true;
+      WidgetsBinding.instance.addPostFrameCallback((_) => _advance());
+    }
+
+    return PopScope(
+      // The notif gate sits between payment-confirmed and the searching
+      // shell. There is nothing meaningful to pop back to (the waiting
+      // screen is terminal once paid), so we swallow the back gesture.
+      canPop: false,
+      child: Scaffold(
+        backgroundColor: HaloTokens.bg,
+        body: SafeArea(
+          child: Padding(
+            padding: const EdgeInsets.fromLTRB(
+              HaloSpacing.s24,
+              HaloSpacing.s32,
+              HaloSpacing.s24,
+              HaloSpacing.s24,
+            ),
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: [
+                const Spacer(),
+                Center(
+                  child: Container(
+                    width: 96,
+                    height: 96,
+                    decoration: const BoxDecoration(
+                      color: HaloTokens.brandSofter,
+                      shape: BoxShape.circle,
+                    ),
+                    child: const Icon(
+                      Icons.notifications_active_outlined,
+                      color: HaloTokens.brandDark,
+                      size: 44,
+                    ),
+                  ),
+                ),
+                const SizedBox(height: HaloSpacing.s24),
+                const Text(
+                  'biar nggak ketinggalan',
+                  textAlign: TextAlign.center,
+                  style: TextStyle(
+                    fontFamily: HaloTokens.fontDisplay,
+                    fontSize: 26,
+                    height: 30 / 26,
+                    fontWeight: FontWeight.w700,
+                    color: HaloTokens.ink,
+                  ),
+                ),
+                const SizedBox(height: HaloSpacing.s12),
+                const Text(
+                  'kami pingin kabarin kamu pas bestie udah siap dengerin. izinin notifikasi biar nggak ada chat yang kelewat.',
+                  textAlign: TextAlign.center,
+                  style: TextStyle(
+                    fontFamily: HaloTokens.fontBody,
+                    fontSize: 14,
+                    height: 20 / 14,
+                    color: HaloTokens.inkSoft,
+                  ),
+                ),
+                const Spacer(),
+                HaloButton(
+                  label: 'izinkan notifikasi',
+                  fullWidth: true,
+                  onPressed: _resolving ? null : _onAllow,
+                ),
+                const SizedBox(height: HaloSpacing.s12),
+                HaloButton(
+                  label: 'nanti aja',
+                  variant: HaloButtonVariant.ghost,
+                  fullWidth: true,
+                  onPressed: _resolving ? null : _onLater,
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}