Phase 4 Stage 7: end-of-session 2-step confirm + thank-you screen

Customer-driven session end flow:
- AppBar 'akhiri' action on chat_screen (visible when connected and
  not already closing).
- Tap fires confirm_end_step1 HaloPopup. lanjut akhiri -> step2;
  gak jadi balik -> dismiss, stay in chat.
- confirm_end_step2 HaloPopup. tulis pesan penutup -> closing_message_sheet
  HaloBottomSheet (textarea + kirim & akhiri / lewat — langsung akhiri).
  lewati saja closes immediately.
- Both close paths POST /api/client/session/:sessionId/end via
  session_closure_notifier.closeSession() and route to /chat/thank-you.
- 409 from the close endpoint surfaces a ClosureRejectedByMitraData
  state and a stub HaloPopup with TODO(stage8) for the BestieOfflinePopup
  returning variant.

Removed the legacy _showSessionExpiredDialog modal — Stage 6's
ChatExpiredBanner is the replacement notification.

Inline _buildGoodbyeView retained with a TODO for the mitra-side early
end flow (still reaches it).

endSessionTwoStepConfirmProvider hardcoded to true with a TODO — the
Stage 1.5 app_config row exists but no client-readable config endpoint
exists yet. Flip the provider to a FutureProvider once the read endpoint
ships.

Maestro 07_end_session_2step.yaml chains after the chat-happy flow
and asserts the Indonesian copy at each step.

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

client_app/lib/core/config/app_config_provider.dart

@@ -0,0 +1,13 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+/// Phase 4 Stage 7 — UX A/B toggle for the two-step end-session confirm.
+///
+/// Backed by `app_config.end_session_two_step_confirm` (seeded `true` in
+/// Phase 4 Stage 1.5). The plan mentions an A/B switch but no client-facing
+/// endpoint is exposed yet — Stage 1.5 only seeded the row. Until a public
+/// `/api/shared/config/app-flags` (or similar) is added, this provider keeps
+/// the seed default on-device. When the endpoint lands, swap the override
+/// for a `FutureProvider` that fetches it.
+///
+/// TODO(phase4-followup): wire to backend once the read-side endpoint is added.
+final endSessionTwoStepConfirmProvider = Provider<bool>((ref) => true);