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/screens/thank_you_screen.dart

@@ -0,0 +1,68 @@
+import 'package:flutter/material.dart';
+import 'package:go_router/go_router.dart';
+import '../../../core/theme/halo_tokens.dart';
+import '../../../core/theme/widgets/halo_button.dart';
+
+/// S11 — landing screen after a session has been closed. Replaces the
+/// previous "navigate straight to home" behavior so the user gets a soft
+/// acknowledgement before re-entering the home shell.
+class ThankYouScreen extends StatelessWidget {
+  const ThankYouScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return PopScope(
+      canPop: false,
+      onPopInvokedWithResult: (didPop, _) {
+        if (!didPop) context.go('/home');
+      },
+      child: Scaffold(
+        backgroundColor: HaloTokens.bg,
+        body: SafeArea(
+          child: Padding(
+            padding: const EdgeInsets.symmetric(horizontal: HaloSpacing.s32),
+            child: Column(
+              mainAxisAlignment: MainAxisAlignment.center,
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: [
+                const Text(
+                  '🤍',
+                  style: TextStyle(fontSize: 72),
+                  textAlign: TextAlign.center,
+                ),
+                const SizedBox(height: HaloSpacing.s24),
+                const Text(
+                  'makasih udah curhat',
+                  style: TextStyle(
+                    fontFamily: HaloTokens.fontDisplay,
+                    fontSize: 26,
+                    fontWeight: FontWeight.w700,
+                    color: HaloTokens.ink,
+                  ),
+                  textAlign: TextAlign.center,
+                ),
+                const SizedBox(height: HaloSpacing.s12),
+                const Text(
+                  'semoga kamu lebih plong sekarang. kalau butuh, bestie selalu siap nemenin lagi.',
+                  style: TextStyle(
+                    fontFamily: HaloTokens.fontBody,
+                    fontSize: 15,
+                    height: 22 / 15,
+                    color: HaloTokens.inkSoft,
+                  ),
+                  textAlign: TextAlign.center,
+                ),
+                const SizedBox(height: HaloSpacing.s40),
+                HaloButton(
+                  label: 'balik ke home',
+                  fullWidth: true,
+                  onPressed: () => context.go('/home'),
+                ),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}