Phase 4 §4: payment-before-pair for returning users + Maestro suite

Stages 5.1, 5.3, 5.4 of the returning-user flow rework. All three §4
entry paths now require payment BEFORE pairing, matching the updated
mermaid spec.

* Spec (requirement/flow_customer.mermaid.md §4): payment block converges
  three call-sites (bestie-yang-udah-kenal-online, bestie-baru,
  offline-popup → cari bestie lain). PairRoute dispatches lama → targeted
  pair, baru/cari-lain → §3 blast. §3 retains its post-payment-shared
  contract.

* Stage 5.1 (client_app): PaymentDraft carries targetedMitraId +
  topicSensitivity. bestie_history_list seeds the draft + pushes
  /payment/entry (was legacy /payment). searching_screen branches on
  draft.targetedMitraId for blast-vs-targeted dispatch.
  payment_entry uses resetExceptTarget(); bestie_choice_sheet + home
  _onCurhatBestieBaruPressed call explicit reset() before push so
  the keepAlive draft can't leak stale targeting into a blast.

* Stage 5.3 (client_app): new BestieOfflineVariant.prePayReturning.
  Bestie-history-list _BestieRow splits tappable from dim so offline
  rows render dimmed but route taps into the popup. CTA "cari bestie
  lain" resets the draft + pushes /payment/entry.

* Stage 5.4 (client_app): deleted legacy /payment route,
  payment_screen.dart, payment_notifier.dart(+.g.dart). router cleaned.

* Tests (requirement/phase4-customer-flow.md + client_app/.maestro/):
  six Maestro flows TS-01..TS-06 covering every §4 branching point,
  all passing end-to-end. Shared onboarding prelude under
  .maestro/subflows/. New helper scripts: accept_latest_pending,
  force_mitra_offline, force_other_mitra_online,
  reset_all_mitras_online, mitra_accept_latest_internal. New backend
  _test endpoints to match. /reset-phone now cascade-deletes
  customer_transactions (FK was blocking). /force-pairing-timeout
  branches targeted (RETURNING_CHAT_TIMEOUT via
  expireTargetedPairingRequest, now exported) vs blast (PAIRING_FAILED).
  seed_history_session also outputs MITRA_NAME_RE (regex-escaped) for
  reliable selectors against display names containing regex specials.

* mitra_app: dispose-during-deactivate guardrail for back-press on the
  mitra chat screen after the customer's goodbye message. Pending real
  emulator repro verification (carried over from 2026-05-15).

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

client_app/lib/router.dart

@@ -14,7 +14,6 @@ import 'features/splash/splash_screen.dart';
 import 'features/home/home_screen.dart';
 import 'features/home/screens/bestie_history_list_screen.dart';
 import 'features/profile/profile_screen.dart';
-import 'core/constants.dart';
 import 'features/chat/screens/searching_screen.dart';
 import 'features/chat/screens/bestie_found_screen.dart';
 import 'features/chat/screens/no_bestie_screen.dart';
@@ -26,7 +25,6 @@ import 'features/chat_tab/screens/pembayaran_view.dart';
 import 'features/chat_tab/screens/selesai_view.dart';
 import 'features/chat/screens/targeted_waiting_screen.dart';
 import 'features/chat/screens/thank_you_screen.dart';
-import 'features/payment/screens/payment_screen.dart';
 import 'features/payment/screens/payment_entry_screen.dart';
 import 'features/payment/screens/discount_paywall_screen.dart';
 import 'features/payment/screens/method_pick_screen.dart';
@@ -166,25 +164,6 @@ GoRouter buildRouter(Ref ref) {
       ),
       GoRoute(path: '/home', builder: (_, __) => const HomeScreen()),
       GoRoute(path: '/profile', builder: (_, __) => const ProfileScreen()),
-      GoRoute(path: '/payment', builder: (context, state) {
-        // Legacy Phase 3.7 single-screen payment. Still reachable from
-        //   - Home "Mulai Curhat" CTA  → no extras (general blast follows confirm)
-        //   - Chat history "Curhat lagi" CTA → extras carry targetedMitraId/mitraName
-        //     for the returning-chat flow, plus optional topicSensitivity.
-        // Phase 4 Stage 3 introduces sibling routes under `/payment/*`; the new
-        // entry point is `/payment/entry`. This route is preserved until Stage 5
-        // migrates the chat-history "Curhat lagi" flow.
-        final extra = state.extra;
-        if (extra is Map<String, dynamic>) {
-          final topic = extra['topicSensitivity'];
-          return PaymentScreen(
-            targetedMitraId: extra['targetedMitraId'] as String?,
-            mitraName: extra['mitraName'] as String?,
-            topicSensitivity: topic is TopicSensitivity ? topic : TopicSensitivity.regular,
-          );
-        }
-        return const PaymentScreen();
-      }),
       // Phase 4 Stage 3 — multi-screen payment shell.
       GoRoute(path: '/payment/entry', builder: (_, __) => const PaymentEntryScreen()),
       GoRoute(path: '/payment/discount-paywall', builder: (_, __) => const DiscountPaywallScreen()),