Phase 4 Stage 10 client_app: Chat tab UI (3 sub-tabs + retire bestie_history)

Flutter half of Stage 10 — the new Chat tab landing in the bottom nav.
The CTA target swaps from /chat/history to /chat, which redirects into
/chat/aktif. Three sibling routes under a single ShellRoute share a
header + sub-tab pills + the existing HaloTabBar footer:

  /chat/aktif        — the current active session (0 or 1 row)
  /chat/pembayaran   — pending initial + extension payments
  /chat/selesai      — past sessions, cursor-paginated infinite scroll

URL is the source of truth for the active sub-tab so deep links, back
stack, and Maestro all agree on state.

New feature dir `lib/features/chat_tab/`:
- providers/pending_payments_provider.dart — FutureProvider against the
  Stage-10 backend endpoint, plus pendingPaymentsCountProvider for the
  red-dot derivative
- providers/selesai_history_provider.dart — AsyncNotifier over
  GET /api/client/chat/history; tracks accumulated items + next_cursor +
  hasMore; loadMore() and refresh()
- widgets/chat_row.dart — generic row used by all 3 sub-tabs, with
  optional PaymentAmountChip / DurationChip / 📞 Call indicator
- widgets/sub_tab_pill.dart — pill with active underline + optional
  numeric badge (null hides; matches Selesai's no-badge rule)
- screens/chat_tab_shell.dart — ShellRoute scaffold + ChatSubTab enum
- screens/{aktif,pembayaran,selesai}_view.dart — the three sub-tab bodies

Router (`router.dart`):
- /chat → redirect → /chat/aktif
- ShellRoute hosts /chat/aktif, /chat/pembayaran, /chat/selesai
- /chat/history retired; /chat/history/:sessionId → /chat/transcript/:sessionId
- ChatHistoryScreen import + file deleted

HaloTabBar (`features/home/widgets/halo_tab_bar.dart` — new in the
working tree from Stage 9 sweep): now a ConsumerWidget. Chat tab goes
to /chat. Red dot renders when pendingPaymentsCountProvider > 0.

Inbound call-site updates:
- bestie_choice_sheet.dart: /chat/history → /chat
- home_screen.dart history-row tap: /chat/history/:id → /chat/transcript/:id

This commit also carries the larger Stage 9 sweep + ESP-removal + USP
gate edits that were already staged in the working tree on
`home_screen.dart` and `router.dart` from the prior session.

flutter analyze: clean except for the pre-existing scaffold
test/widget_test.dart MyApp reference (unrelated, present on master).

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

client_app/lib/features/chat_tab/screens/aktif_view.dart

@@ -0,0 +1,111 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
+import '../../../core/chat/active_session_notifier.dart';
+import '../../../core/theme/halo_tokens.dart';
+import '../widgets/chat_row.dart';
+
+/// Chat-Tab > aktif sub-tab.
+///
+/// Always renders 0 or 1 row — backend caps the customer to one active
+/// session. Row stays visible while the user is inside the live chat room
+/// (per §10.6 decision 1: aktif represents state, not navigation).
+class AktifView extends ConsumerWidget {
+  const AktifView({super.key});
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final async = ref.watch(activeSessionProvider);
+    return RefreshIndicator(
+      onRefresh: () => ref.read(activeSessionProvider.notifier).refresh(),
+      color: HaloTokens.brand,
+      child: async.when(
+        loading: () => const _Loading(),
+        error: (e, _) => _Error(message: e.toString()),
+        data: (data) {
+          if (!data.hasSession) return const _Empty();
+          final session = data.session!;
+          final mitraName =
+              (session['mitra_display_name'] as String?) ?? 'Bestie';
+          return ListView(
+            padding: const EdgeInsets.symmetric(
+              horizontal: HaloSpacing.s20,
+              vertical: HaloSpacing.s12,
+            ),
+            children: [
+              ChatRow(
+                seed: mitraName.codeUnits.fold<int>(0, (a, b) => a + b),
+                name: mitraName,
+                preview: _previewFor(data.unreadCount),
+                isLive: true,
+                isCall: (session['mode'] as String?) == 'call',
+                onTap: () {
+                  final sessionId = session['id'] as String?;
+                  if (sessionId == null) return;
+                  context.push('/chat/session/$sessionId',
+                      extra: {'mitraName': mitraName});
+                },
+              ),
+            ],
+          );
+        },
+      ),
+    );
+  }
+
+  String _previewFor(int unread) =>
+      unread > 0 ? '$unread pesan baru' : 'lagi ngobrol nih';
+}
+
+class _Empty extends StatelessWidget {
+  const _Empty();
+  @override
+  Widget build(BuildContext context) => const _CenteredMessage(
+        text: 'belum ada chat di sini',
+      );
+}
+
+class _Loading extends StatelessWidget {
+  const _Loading();
+  @override
+  Widget build(BuildContext context) => const Center(
+        child: CircularProgressIndicator(color: HaloTokens.brand),
+      );
+}
+
+class _Error extends StatelessWidget {
+  final String message;
+  const _Error({required this.message});
+  @override
+  Widget build(BuildContext context) =>
+      _CenteredMessage(text: 'gagal memuat: $message');
+}
+
+class _CenteredMessage extends StatelessWidget {
+  final String text;
+  const _CenteredMessage({required this.text});
+  @override
+  Widget build(BuildContext context) {
+    // Wrap in a scroll view so RefreshIndicator works even on empty state.
+    return ListView(
+      physics: const AlwaysScrollableScrollPhysics(),
+      children: [
+        Padding(
+          padding: const EdgeInsets.symmetric(
+            horizontal: HaloSpacing.s20,
+            vertical: HaloSpacing.s64,
+          ),
+          child: Text(
+            text,
+            textAlign: TextAlign.center,
+            style: const TextStyle(
+              fontFamily: HaloTokens.fontBody,
+              fontSize: 13,
+              color: HaloTokens.inkMuted,
+            ),
+          ),
+        ),
+      ],
+    );
+  }
+}