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/features/chat/widgets/confirm_end_step2.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/material.dart';
+import '../../../core/theme/widgets/halo_popup.dart';
+
+/// Stage 7 — second confirm popup. Customer has already chosen to end; this
+/// step nudges (not forces) them to leave a closing message, with `lewati saja`
+/// as the bypass into the close-session API directly.
+class ConfirmEndStep2 {
+  const ConfirmEndStep2._();
+
+  static Future<void> show(
+    BuildContext context, {
+    required VoidCallback onWriteMessage,
+    required VoidCallback onSkip,
+  }) {
+    return HaloPopup.show<void>(
+      context,
+      title: 'mau tinggalin pesan penutup?',
+      body: 'kamu bisa tulis pesan terakhir buat bestie sebelum sesi ditutup',
+      icon: const Text('💌', style: TextStyle(fontSize: 40)),
+      primary: HaloPopupAction(
+        label: 'tulis pesan penutup',
+        onPressed: onWriteMessage,
+      ),
+      secondary: HaloPopupAction(label: 'lewati saja', onPressed: onSkip),
+    );
+  }
+}