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_step1.dart

@@ -0,0 +1,26 @@
+import 'package:flutter/material.dart';
+import '../../../core/theme/widgets/halo_popup.dart';
+
+/// Stage 7 — first of two confirm popups before ending the session. Surface
+/// the soft "balik" exit prominently because the most common path here is the
+/// user mis-tapping "akhiri sesi" while still wanting to continue.
+class ConfirmEndStep1 {
+  const ConfirmEndStep1._();
+
+  static Future<void> show(
+    BuildContext context, {
+    required VoidCallback onConfirm,
+  }) {
+    return HaloPopup.show<void>(
+      context,
+      title: 'yakin mau akhiri sesi?',
+      body: 'sesi akan ditutup dan kamu balik ke home',
+      icon: const Text('🤔', style: TextStyle(fontSize: 40)),
+      primary: HaloPopupAction(label: 'lanjut akhiri', onPressed: onConfirm),
+      secondary: HaloPopupAction(
+        label: 'gak jadi, balik',
+        onPressed: () {},
+      ),
+    );
+  }
+}