Phase 4 Stage 6: chat-room countdown UX + voice-call mode pill

Customer chat screen:
- Voice-call header pill (mode == 'call' renders accent-colored pill;
  chat mode renders no pill).
- HaloSnackbar fires once per session at 180s remaining ('sisa 3 menit
  lagi ya 🤍'), driven by the backend session_warning WS event.
- Last-2-min danger styling: timer pill flips to HaloTokens.danger +
  bold JetBrainsMono when remaining <= 120s.
- Floating ChatExpiredBanner widget injected above the input bar when
  remaining hits 0 in closing-grace state. perpanjang -> existing
  pricing bottom sheet.
- pricing_bottom_sheet.dart rewritten to the 5-option layout with
  chat|call mode toggle (mirrors duration-pick from Stage 3).

Mitra chat screen: voice-call header pill only (no countdown UX per PRD).

Backend:
- session.service.js getSessionById JOINs payment_sessions so mode +
  expires_at ship in /api/shared/chat/:id/info.
- session-timer.service.js onThreeMinuteWarning now emits expires_at +
  remaining_seconds for client resync.
- Dev-only POST /internal/_test/force-session-expires-at clears the
  3-min flag, reschedules the timer, and broadcasts WS resync. Lets
  the Maestro flow drive 175s -> 90s -> 0s without waiting live.

New chatRemainingSeconds StreamProvider derived from expiresAt, fed by
session_warning / session_timer / session_expired resync messages
(plan referenced a secondsLeftProvider that didn't actually exist).

Maestro 06_chat_countdown.yaml + force_session_expires_at.js helper.

Out of scope: meet.google.com URL launching - url_launcher isn't a
client_app dependency and message bubbles render plain Text. Defer.

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

client_app/lib/features/chat/widgets/chat_expired_banner.dart

@@ -0,0 +1,59 @@
+import 'package:flutter/material.dart';
+import '../../../core/theme/halo_tokens.dart';
+import '../../../core/theme/widgets/halo_button.dart';
+
+/// Floating banner injected above the chat input bar when the session timer
+/// has hit zero but the session is still in `closing` grace. Phase 4 Stage 6:
+/// gives the customer a soft, in-place way to extend instead of the modal-only
+/// flow from Phase 3.
+class ChatExpiredBanner extends StatelessWidget {
+  final VoidCallback onExtend;
+
+  const ChatExpiredBanner({super.key, required this.onExtend});
+
+  @override
+  Widget build(BuildContext context) {
+    return Container(
+      margin: const EdgeInsets.fromLTRB(
+        HaloSpacing.s12,
+        HaloSpacing.s8,
+        HaloSpacing.s12,
+        HaloSpacing.s8,
+      ),
+      padding: const EdgeInsets.fromLTRB(
+        HaloSpacing.s16,
+        HaloSpacing.s12,
+        HaloSpacing.s12,
+        HaloSpacing.s12,
+      ),
+      decoration: const BoxDecoration(
+        color: HaloTokens.danger,
+        borderRadius: HaloRadius.lg,
+        boxShadow: HaloShadows.card,
+      ),
+      child: Row(
+        children: [
+          const Text('⏰', style: TextStyle(fontSize: 20)),
+          const SizedBox(width: HaloSpacing.s12),
+          const Expanded(
+            child: Text(
+              'waktu curhat habis',
+              style: TextStyle(
+                fontFamily: HaloTokens.fontBody,
+                fontSize: 14,
+                fontWeight: FontWeight.w600,
+                color: Colors.white,
+              ),
+            ),
+          ),
+          HaloButton(
+            label: 'perpanjang',
+            size: HaloButtonSize.sm,
+            variant: HaloButtonVariant.secondary,
+            onPressed: onExtend,
+          ),
+        ],
+      ),
+    );
+  }
+}