test: add golden tests for key EmailListScreen states (T5)

Five golden tests covering: empty inbox, populated list (unread/flagged),
selection mode, search results, and error banner. Goldens are generated
on the same Linux environment as CI so rasterisation is deterministic.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/widget/email_list_screen_golden_test.dart

@@ -0,0 +1,158 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:sharedinbox/core/models/email.dart';
+import 'package:sharedinbox/di.dart';
+
+import 'helpers.dart';
+
+// Fixed-date emails so golden files don't change day to day.
+final _kDate = DateTime(2024, 6);
+
+Email _email({
+  String id = 'acc-1:1',
+  String subject = 'Hello world',
+  bool isSeen = true,
+  bool isFlagged = false,
+}) =>
+    Email(
+      id: id,
+      accountId: 'acc-1',
+      mailboxPath: 'INBOX',
+      uid: int.parse(id.split(':').last),
+      subject: subject,
+      receivedAt: _kDate,
+      sentAt: _kDate,
+      from: const [EmailAddress(name: 'Bob', email: 'bob@example.com')],
+      to: const [EmailAddress(email: 'alice@example.com')],
+      cc: const [],
+      isSeen: isSeen,
+      isFlagged: isFlagged,
+      hasAttachment: false,
+    );
+
+List<Override> _overrides({
+  List<Email> emails = const [],
+  List<Email> searchResults = const [],
+  String? syncError,
+}) =>
+    [
+      accountRepositoryProvider.overrideWithValue(
+        FakeAccountRepository([kTestAccount]),
+      ),
+      mailboxRepositoryProvider.overrideWithValue(
+        FakeMailboxRepository([kTestMailbox]),
+      ),
+      emailRepositoryProvider.overrideWithValue(
+        FakeEmailRepository(emails: emails, searchResults: searchResults),
+      ),
+      draftRepositoryProvider.overrideWithValue(FakeDraftRepository()),
+      searchHistoryRepositoryProvider.overrideWithValue(
+        FakeSearchHistoryRepository(),
+      ),
+      syncLastErrorProvider.overrideWith(
+        (ref, _) => Stream.value(syncError),
+      ),
+    ];
+
+void main() {
+  group('EmailListScreen goldens', () {
+    testWidgets('golden: empty state', (tester) async {
+      await tester.pumpWidget(
+        buildApp(
+          initialLocation: '/accounts/acc-1/mailboxes/INBOX/emails',
+          overrides: _overrides(),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await expectLater(
+        find.byType(MaterialApp),
+        matchesGoldenFile('goldens/email_list_empty.png'),
+      );
+    });
+
+    testWidgets('golden: list with emails', (tester) async {
+      await tester.pumpWidget(
+        buildApp(
+          initialLocation: '/accounts/acc-1/mailboxes/INBOX/emails',
+          overrides: _overrides(
+            emails: [
+              _email(subject: 'Team standup notes', isSeen: false),
+              _email(id: 'acc-1:2', subject: 'Q3 review', isFlagged: true),
+              _email(id: 'acc-1:3', subject: 'Welcome to the project'),
+            ],
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await expectLater(
+        find.byType(MaterialApp),
+        matchesGoldenFile('goldens/email_list_with_emails.png'),
+      );
+    });
+
+    testWidgets('golden: selection mode', (tester) async {
+      await tester.pumpWidget(
+        buildApp(
+          initialLocation: '/accounts/acc-1/mailboxes/INBOX/emails',
+          overrides: _overrides(
+            emails: [
+              _email(subject: 'Team standup notes', isSeen: false),
+              _email(id: 'acc-1:2', subject: 'Q3 review'),
+            ],
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await tester.longPress(find.text('Team standup notes'));
+      await tester.pumpAndSettle();
+
+      await expectLater(
+        find.byType(MaterialApp),
+        matchesGoldenFile('goldens/email_list_selection.png'),
+      );
+    });
+
+    testWidgets('golden: search with results', (tester) async {
+      await tester.pumpWidget(
+        buildApp(
+          initialLocation: '/accounts/acc-1/mailboxes/INBOX/emails',
+          overrides: _overrides(
+            searchResults: [
+              _email(id: 'acc-1:5', subject: 'Project proposal'),
+            ],
+          ),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await tester.enterText(find.byType(SearchBar), 'project');
+      await tester.testTextInput.receiveAction(TextInputAction.search);
+      await tester.pumpAndSettle();
+
+      await expectLater(
+        find.byType(MaterialApp),
+        matchesGoldenFile('goldens/email_list_search_results.png'),
+      );
+    });
+
+    testWidgets('golden: error banner', (tester) async {
+      await tester.pumpWidget(
+        buildApp(
+          initialLocation: '/accounts/acc-1/mailboxes/INBOX/emails',
+          overrides: _overrides(syncError: 'Connection refused'),
+        ),
+      );
+      await tester.pumpAndSettle();
+
+      await expectLater(
+        find.byType(MaterialApp),
+        matchesGoldenFile('goldens/email_list_error_banner.png'),
+      );
+    });
+  });
+}