feat(undo-log): hyperlink email rows in Undo Log Detail (#474) (#547)

## Summary

- Each email row in the **Undo Log Detail** "Emails" section is now tappable.
- Tapping resolves the email via `EmailRepository.findEmailByMessageId(accountId, messageId)` and navigates to its **current** location, so the link survives the move/snooze that changed its IMAP UID.
- If the email has no Message-ID, or no row matches the lookup (e.g. hard-deleted), a SnackBar explains the situation instead of navigating.

A `chevron_right` trailing icon was added to signal the rows are now navigable.

Closes #474

## Test plan

- [x] New widget test `test/widget/undo_log_detail_screen_test.dart` covers:
  - tap on a row whose lookup hits → navigates to `/accounts/<acc>/mailboxes/<encoded>/emails/<encoded>` with the **current** mailbox/id
  - tap when lookup returns `null` → "Email no longer exists" SnackBar, no navigation
  - tap when the original row has no Message-ID → "no Message-ID" SnackBar, no navigation

Co-authored-by: guettli <guettli@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/547

test/widget/undo_log_detail_screen_test.dart

@@ -0,0 +1,176 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:go_router/go_router.dart';
+
+import 'package:sharedinbox/core/models/email.dart';
+import 'package:sharedinbox/core/models/undo_action.dart';
+import 'package:sharedinbox/di.dart';
+import 'package:sharedinbox/ui/screens/undo_log_detail_screen.dart';
+
+import 'helpers.dart';
+
+// FakeEmailRepository subclass that returns a pre-configured email from
+// findEmailByMessageId, so the tap handler in UndoLogDetailScreen can be
+// exercised without a real database.
+class _LookupEmailRepository extends FakeEmailRepository {
+  _LookupEmailRepository(this._lookup);
+
+  final Email? _lookup;
+
+  @override
+  Future<Email?> findEmailByMessageId(
+    String accountId,
+    String messageId,
+  ) async =>
+      _lookup;
+}
+
+UndoAction _action({
+  required List<Email> originalEmails,
+  String accountId = 'acc-1',
+}) =>
+    UndoAction(
+      id: 'undo-1',
+      accountId: accountId,
+      type: UndoType.move,
+      emailIds: originalEmails.map((e) => e.id).toList(),
+      sourceMailboxPath: 'INBOX',
+      destinationMailboxPath: 'Archive',
+      originalEmails: originalEmails,
+      timestamp: DateTime(2024, 6),
+    );
+
+Email _emailWith({
+  String id = 'acc-1:42',
+  String mailboxPath = 'INBOX',
+  String? messageId = '<msg-1@example.com>',
+}) =>
+    Email(
+      id: id,
+      accountId: 'acc-1',
+      mailboxPath: mailboxPath,
+      uid: 42,
+      subject: 'Hello world',
+      receivedAt: DateTime(2024, 6),
+      sentAt: DateTime(2024, 6),
+      from: const [EmailAddress(name: 'Bob', email: 'bob@example.com')],
+      to: const [EmailAddress(email: 'alice@example.com')],
+      cc: const [],
+      isSeen: false,
+      isFlagged: false,
+      hasAttachment: false,
+      messageId: messageId,
+    );
+
+// Builds a minimal app whose initial location is the undo log detail screen
+// for [action]. A placeholder email-detail route records its visit so the
+// test can assert which path the tap navigated to.
+Widget _buildApp({
+  required UndoAction action,
+  required FakeEmailRepository emailRepo,
+  ValueNotifier<String?>? lastEmailRoute,
+}) {
+  final router = GoRouter(
+    initialLocation: '/undo-detail',
+    routes: [
+      GoRoute(
+        path: '/undo-detail',
+        builder: (ctx, state) => UndoLogDetailScreen(action: action),
+      ),
+      GoRoute(
+        path: '/accounts/:accountId/mailboxes/:mailboxPath/emails/:emailId',
+        builder: (ctx, state) {
+          lastEmailRoute?.value = state.uri.toString();
+          return const Scaffold(body: Text('email-detail-route'));
+        },
+      ),
+    ],
+  );
+
+  return ProviderScope(
+    overrides: [
+      emailRepositoryProvider.overrideWithValue(emailRepo),
+    ],
+    child: MaterialApp.router(routerConfig: router),
+  );
+}
+
+void main() {
+  group('UndoLogDetailScreen email row tap', () {
+    testWidgets('navigates to the current location returned by lookup', (
+      tester,
+    ) async {
+      // Original row recorded INBOX/42; after the move it now lives in
+      // Archive with a fresh UID — the lookup is what bridges that gap.
+      final original = _emailWith();
+      final current = _emailWith(id: 'acc-1:77', mailboxPath: 'Archive');
+      final lastRoute = ValueNotifier<String?>(null);
+
+      await tester.pumpWidget(
+        _buildApp(
+          action: _action(originalEmails: [original]),
+          emailRepo: _LookupEmailRepository(current),
+          lastEmailRoute: lastRoute,
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await tester.tap(find.text('Hello world'));
+      await tester.pumpAndSettle();
+
+      expect(find.text('email-detail-route'), findsOneWidget);
+      expect(
+        lastRoute.value,
+        '/accounts/acc-1/mailboxes/Archive/emails/acc-1%3A77',
+      );
+    });
+
+    testWidgets('shows snackbar when lookup returns null', (tester) async {
+      final original = _emailWith();
+      final lastRoute = ValueNotifier<String?>(null);
+
+      await tester.pumpWidget(
+        _buildApp(
+          action: _action(originalEmails: [original]),
+          emailRepo: _LookupEmailRepository(null),
+          lastEmailRoute: lastRoute,
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await tester.tap(find.text('Hello world'));
+      await tester.pump();
+
+      expect(
+        find.textContaining('Email no longer exists'),
+        findsOneWidget,
+      );
+      expect(lastRoute.value, isNull);
+      expect(find.text('email-detail-route'), findsNothing);
+    });
+
+    testWidgets('shows snackbar when email has no Message-ID', (tester) async {
+      final original = _emailWith(messageId: null);
+      final lastRoute = ValueNotifier<String?>(null);
+
+      await tester.pumpWidget(
+        _buildApp(
+          action: _action(originalEmails: [original]),
+          // Lookup would succeed if called, but with no Message-ID the
+          // tap handler must short-circuit before reaching it.
+          emailRepo: _LookupEmailRepository(_emailWith()),
+          lastEmailRoute: lastRoute,
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await tester.tap(find.text('Hello world'));
+      await tester.pump();
+
+      expect(find.textContaining('no Message-ID'), findsOneWidget);
+      expect(lastRoute.value, isNull);
+      expect(find.text('email-detail-route'), findsNothing);
+    });
+  });
+}