Phase 4 Stage 3: payment shell (multi-screen flow)

Six new screens under /payment/* + a paymentDraftProvider holding
mode/durationId/durationMinutes/priceIDR/paymentId/isFirstSessionDiscount
across the flow. PaymentEntryScreen handles the routing decision
(eligible+enabled -> /payment/discount-paywall, else /payment/method-pick)
and clears the draft on fresh entry.

Screens:
- discount_paywall_screen: S6 first-session discount with struck-through
  gimmick price + actual price + 'mulai · Rp{actual}' CTA -> /payment/method
- method_pick_screen: chat vs call cards
- duration_pick_screen: tier list with chat|call mode toggle that resets
  the selection on swap
- payment_method_screen: QRIS-first list, posts to existing
  /api/client/payment-sessions with mode/duration/price/discount/method
- waiting_payment_screen: qr_flutter QR (encodes paymentId in mock mode),
  20-min countdown header, 3s polling for status, pauses on background
  via WidgetsBindingObserver
- payment_expired_screen: retry CTA -> /payment/method with draft retained

Status mapping: real payment_sessions.status uses 'confirmed'/'consumed'
for paid (not 'paid' as in plan) and 'expired'/'abandoned' as terminal.

home_screen 'Mulai Curhat' CTA now pushes /payment/entry.

Dev-only /internal/_test/force-expire-payment endpoint to drive Maestro
flow 04_payment_expired.yaml without waiting 20 minutes. Gated behind
NODE_ENV !== 'production'.

chat_opening_provider PricingData extended to carry Phase 4 chat/call
groups + firstSessionDiscount, back-compat with the Phase 3 shape.

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

client_app/lib/features/payment/payment_notifier.g.dart

@@ -6,7 +6,7 @@ part of 'payment_notifier.dart';
 // RiverpodGenerator
 // **************************************************************************
 
-String _$paymentHash() => r'63019ba794311cd36761bd6ad6f90b0abde5c747';
+String _$paymentHash() => r'd98f2e7e5045ea2a39b7af0d4a9f0601dd06ce74';
 
 /// See also [Payment].
 @ProviderFor(Payment)