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/pubspec.lock

@@ -125,10 +125,10 @@ packages:
     dependency: transitive
     description:
       name: characters
-      sha256: f71061c654a3380576a52b451dd5532377954cf9dbd272a78fc8479606670803
+      sha256: faf38497bda5ead2a8c7615f4f7939df04333478bf32e4173fcb06d428b5716b
       url: "https://pub.dev"
     source: hosted
-    version: "1.4.0"
+    version: "1.4.1"
   checked_yaml:
     dependency: transitive
     description:
@@ -691,18 +691,18 @@ packages:
     dependency: transitive
     description:
       name: matcher
-      sha256: dc58c723c3c24bf8d3e2d3ad3f2f9d7bd9cf43ec6feaa64181775e60190153f2
+      sha256: dc0b7dc7651697ea4ff3e69ef44b0407ea32c487a39fff6a4004fa585e901861
       url: "https://pub.dev"
     source: hosted
-    version: "0.12.17"
+    version: "0.12.19"
   material_color_utilities:
     dependency: transitive
     description:
       name: material_color_utilities
-      sha256: f7142bb1154231d7ea5f96bc7bde4bda2a0945d2806bb11670e30b850d56bdec
+      sha256: "9c337007e82b1889149c82ed242ed1cb24a66044e30979c44912381e9be4c48b"
       url: "https://pub.dev"
     source: hosted
-    version: "0.11.1"
+    version: "0.13.0"
   meta:
     dependency: transitive
     description:
@@ -1136,10 +1136,10 @@ packages:
     dependency: transitive
     description:
       name: test_api
-      sha256: ab2726c1a94d3176a45960b6234466ec367179b87dd74f1611adb1f3b5fb9d55
+      sha256: "8161c84903fd860b26bfdefb7963b3f0b68fee7adea0f59ef805ecca346f0c7a"
       url: "https://pub.dev"
     source: hosted
-    version: "0.7.7"
+    version: "0.7.10"
   timezone:
     dependency: transitive
     description: