fix(order): back from invoice screens lands on trade detail

lib/features/order/screens/add_lightning_invoice_screen.dart

@@ -66,6 +66,49 @@ class _AddLightningInvoiceScreenState
     return true;
   }
 
+  /// Cancel button = cancel the trade itself (confirmed via dialog), not
+  /// just leave the screen — going back is what lands on trade detail (#268).
+  Future<void> _cancelOrder() async {
+    final l10n = AppLocalizations.of(context);
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: Text(l10n.cancelTradeDialogTitle),
+        content: Text(l10n.cancelTradeDialogContent),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, false),
+            child: Text(l10n.noButtonLabel),
+          ),
+          FilledButton(
+            onPressed: () => Navigator.pop(ctx, true),
+            child: Text(l10n.yesCancelButtonLabel),
+          ),
+        ],
+      ),
+    );
+    if (!mounted || confirmed != true) return;
+    try {
+      await orders_api.cancelOrder(orderId: widget.orderId);
+      if (!mounted) return;
+      _navigated = true;
+      refreshTrades(ref);
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text(l10n.cancelRequestSent)),
+      );
+      context.go(AppRoute.home);
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text(
+            localizedDaemonError(l10n, e, fallback: l10n.cancelRequestFailed),
+          ),
+        ),
+      );
+    }
+  }
+
   Future<void> _submit(WidgetRef ref) async {
     if (_submitting) return;
     final input = _invoiceController.text.trim();
@@ -271,7 +314,7 @@ class _AddLightningInvoiceScreenState
               children: [
                 Expanded(
                   child: TextButton(
-                    onPressed: () => context.pop(),
+                    onPressed: _cancelOrder,
                     child: Text(
                       l10n.cancel,
                       style: TextStyle(color: colors?.textSecondary),

lib/features/order/screens/pay_lightning_invoice_screen.dart

@@ -8,11 +8,13 @@ import 'package:url_launcher/url_launcher.dart';
 
 import 'package:mostro/core/app_routes.dart';
 import 'package:mostro/core/app_theme.dart';
+import 'package:mostro/core/daemon_errors.dart';
 import 'package:mostro/features/order/providers/trade_state_provider.dart';
 import 'package:mostro/features/settings/providers/nwc_provider.dart';
 import 'package:mostro/features/trades/providers/trades_providers.dart'
     show refreshTrades;
 import 'package:mostro/l10n/app_localizations.dart';
+import 'package:mostro/src/rust/api/orders.dart' as orders_api;
 import 'package:mostro/src/rust/api/types.dart' show OrderStatus, TradeUpdate;
 import 'package:mostro/shared/widgets/nwc_payment_widget.dart';
 
@@ -46,6 +48,49 @@ class _PayLightningInvoiceScreenState
     setState(() => _waiting = true);
   }
 
+  /// Cancel button = cancel the trade itself (confirmed via dialog), not
+  /// just leave the screen — going back is what lands on trade detail (#268).
+  Future<void> _cancelOrder() async {
+    final l10n = AppLocalizations.of(context);
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: Text(l10n.cancelTradeDialogTitle),
+        content: Text(l10n.cancelTradeDialogContent),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, false),
+            child: Text(l10n.noButtonLabel),
+          ),
+          FilledButton(
+            onPressed: () => Navigator.pop(ctx, true),
+            child: Text(l10n.yesCancelButtonLabel),
+          ),
+        ],
+      ),
+    );
+    if (!mounted || confirmed != true) return;
+    try {
+      await orders_api.cancelOrder(orderId: widget.orderId);
+      if (!mounted) return;
+      _navigated = true;
+      refreshTrades(ref);
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(content: Text(l10n.cancelRequestSent)),
+      );
+      context.go(AppRoute.home);
+    } catch (e) {
+      if (!mounted) return;
+      ScaffoldMessenger.of(context).showSnackBar(
+        SnackBar(
+          content: Text(
+            localizedDaemonError(l10n, e, fallback: l10n.cancelRequestFailed),
+          ),
+        ),
+      );
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final theme = Theme.of(context);
@@ -364,7 +409,7 @@ class _PayLightningInvoiceScreenState
                   SizedBox(
                     width: double.infinity,
                     child: OutlinedButton(
-                      onPressed: () => context.pop(),
+                      onPressed: _cancelOrder,
                       style: OutlinedButton.styleFrom(
                         foregroundColor:
                             colors?.destructiveRed ?? const Color(0xFFD84D4D),

lib/features/order/screens/take_order_screen.dart

@@ -49,6 +49,10 @@ class _TakeOrderScreenState extends ConsumerState<TakeOrderScreen> {
   @override
   void initState() {
     super.initState();
+    // Defense in depth (#268): if the user already participates in this
+    // order (deep link, stale book entry, back navigation), Take Order
+    // must not offer to take it again — land on the trade instead.
+    _redirectIfParticipant();
     // Try immediately in case the provider already has data.
     _tryStartCountdown();
     // If the provider is still loading, listen for the first value.
@@ -58,6 +62,12 @@ class _TakeOrderScreenState extends ConsumerState<TakeOrderScreen> {
     });
   }
 
+  Future<void> _redirectIfParticipant() async {
+    final role = await orders_api.getTradeRole(orderId: widget.orderId);
+    if (!mounted || role == null) return;
+    context.go(AppRoute.tradeDetailPath(widget.orderId));
+  }
+
   void _tryStartCountdown() {
     if (_countdownTimer != null) return; // already running
     _startCountdown();
@@ -142,9 +152,15 @@ class _TakeOrderScreenState extends ConsumerState<TakeOrderScreen> {
           // LN address was included in take-sell payload — go straight to trade.
           context.go(AppRoute.tradeDetailPath(widget.orderId));
         } else {
+          // Rebuild the stack with trade detail as the base so back/close
+          // from add-invoice lands on the trade, never back on Take Order
+          // offering to take an already-taken order (#268).
+          context.go(AppRoute.tradeDetailPath(widget.orderId));
           context.push(AppRoute.addInvoicePath(widget.orderId));
         }
       } else {
+        // Same stack shape for the seller's pay-invoice screen (#268).
+        context.go(AppRoute.tradeDetailPath(widget.orderId));
         context.push(AppRoute.payInvoicePath(widget.orderId));
       }
     } catch (e) {