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/closing_message_sheet.dart

@@ -0,0 +1,138 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import '../../../core/chat/session_closure_notifier.dart';
+import '../../../core/theme/halo_tokens.dart';
+import '../../../core/theme/widgets/halo_bottom_sheet.dart';
+import '../../../core/theme/widgets/halo_button.dart';
+
+/// Stage 7 — replaces the legacy goodbye-composer screen with a bottom sheet.
+/// The sheet is launched after the two-step confirm; it submits the goodbye
+/// message AND closes the session. Both CTAs end the session — the difference
+/// is whether a closing message is sent first.
+class ClosingMessageSheet {
+  const ClosingMessageSheet._();
+
+  static Future<void> show(
+    BuildContext context, {
+    required String sessionId,
+    required VoidCallback onCompleted,
+  }) {
+    return HaloBottomSheet.show<void>(
+      context,
+      isScrollControlled: true,
+      child: _ClosingMessageBody(
+        sessionId: sessionId,
+        onCompleted: onCompleted,
+      ),
+    );
+  }
+}
+
+class _ClosingMessageBody extends ConsumerStatefulWidget {
+  final String sessionId;
+  final VoidCallback onCompleted;
+
+  const _ClosingMessageBody({
+    required this.sessionId,
+    required this.onCompleted,
+  });
+
+  @override
+  ConsumerState<_ClosingMessageBody> createState() =>
+      _ClosingMessageBodyState();
+}
+
+class _ClosingMessageBodyState extends ConsumerState<_ClosingMessageBody> {
+  final _controller = TextEditingController();
+  bool _busy = false;
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  Future<void> _sendAndEnd() async {
+    final text = _controller.text.trim();
+    if (text.isEmpty || _busy) return;
+    setState(() => _busy = true);
+    final notifier = ref.read(sessionClosureProvider.notifier);
+    await notifier.submitGoodbye(widget.sessionId, text);
+    await notifier.closeSession(widget.sessionId);
+    if (!mounted) return;
+    Navigator.of(context).pop();
+    widget.onCompleted();
+  }
+
+  Future<void> _skipAndEnd() async {
+    if (_busy) return;
+    setState(() => _busy = true);
+    await ref.read(sessionClosureProvider.notifier).closeSession(widget.sessionId);
+    if (!mounted) return;
+    Navigator.of(context).pop();
+    widget.onCompleted();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final viewInsets = MediaQuery.of(context).viewInsets.bottom;
+    return Padding(
+      padding: EdgeInsets.only(bottom: viewInsets),
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          const Text(
+            'pesan penutup',
+            style: TextStyle(
+              fontFamily: HaloTokens.fontDisplay,
+              fontSize: 20,
+              fontWeight: FontWeight.w700,
+              color: HaloTokens.ink,
+            ),
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: HaloSpacing.s8),
+          const Text(
+            'tulis sesuatu buat bestie sebelum sesi ditutup',
+            style: TextStyle(
+              fontFamily: HaloTokens.fontBody,
+              fontSize: 14,
+              color: HaloTokens.inkSoft,
+            ),
+            textAlign: TextAlign.center,
+          ),
+          const SizedBox(height: HaloSpacing.s16),
+          TextField(
+            controller: _controller,
+            maxLines: 4,
+            minLines: 3,
+            enabled: !_busy,
+            decoration: const InputDecoration(
+              hintText: 'makasih ya bestie...',
+              filled: true,
+              fillColor: HaloTokens.brandSofter,
+              border: OutlineInputBorder(
+                borderRadius: HaloRadius.lg,
+                borderSide: BorderSide.none,
+              ),
+            ),
+          ),
+          const SizedBox(height: HaloSpacing.s16),
+          HaloButton(
+            label: 'kirim & akhiri sesi',
+            fullWidth: true,
+            onPressed: _busy ? null : _sendAndEnd,
+          ),
+          const SizedBox(height: HaloSpacing.s8),
+          HaloButton(
+            label: 'lewat — langsung akhiri',
+            variant: HaloButtonVariant.ghost,
+            fullWidth: true,
+            onPressed: _busy ? null : _skipAndEnd,
+          ),
+        ],
+      ),
+    );
+  }
+}