fix(detail): auto-dismiss "Load remote images" snack bar (#548)

## Summary

- The "Load remote images" snack bar in single-mail view (and the analogous thread view) never disappeared on its own — the user had to interact with it.
- Flutter's `SnackBar` defaults to `persist: true` whenever an `action` is provided (see `flutter/lib/src/material/snack_bar.dart`: `persist = persist ?? action != null`), which short-circuits the duration-based dismiss timer in `ScaffoldMessengerState.build`:

  ```dart
  _snackBarTimer = Timer(snackBar.duration, () {
    if (snackBar.persist) return;          // <-- here
    hideCurrentSnackBar(reason: SnackBarClosedReason.timeout);
  });
  ```

  So the explicit `duration: 3s` was set, but the "View" action made the snack bar persistent and the timer's callback returned early.
- Pass `persist: false` explicitly on both snack bars so the 3-second timer fires and the snack bar slides away on its own, while the "View" action button still works to navigate to the trusted-senders settings.

## Test plan

- [x] Added widget regression test in `test/widget/email_detail_screen_test.dart` (`Load remote images snack bar auto-dismisses after 3 seconds`).
- [x] Added analogous test in `test/widget/thread_detail_screen_test.dart`.
- [x] `task test-widget` — all 174 widget tests pass.
- [x] `scripts/run_unit_tests.sh` — all 552 unit tests pass.
- [x] `fvm dart analyze --fatal-infos` on changed files — no issues.
- [x] `fvm dart format` — no diffs.
- [ ] Manual: open a single mail with HTML body from an untrusted sender; tap "Load remote images"; verify the snack bar appears, images load, and the snack bar disappears after ~3 seconds while the "View" action button still navigates to `/accounts/trusted-senders` when tapped.

Closes #484

Co-authored-by: Agentloop Bot <agentloop-bot@noreply.codeberg.org>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/548

test/widget/email_detail_screen_test.dart

@@ -582,6 +582,54 @@ void main() {
 
       expect(find.textContaining('Structure not available'), findsOneWidget);
     });
+
+    testWidgets(
+      'Load remote images snack bar auto-dismisses after 3 seconds',
+      (tester) async {
+        const body = EmailBody(
+          emailId: 'acc-1:42',
+          htmlBody: '<p>Hello <img src="https://example.com/x.png"/></p>',
+          attachments: [],
+        );
+        await tester.pumpWidget(
+          buildApp(
+            initialLocation:
+                '/accounts/acc-1/mailboxes/INBOX/emails/acc-1%3A42',
+            overrides: _overrides(body: body),
+          ),
+        );
+        await tester.pumpAndSettle();
+
+        // The "Load remote images" button is visible because the sender is
+        // not yet trusted.
+        expect(find.text('Load remote images'), findsOneWidget);
+
+        await tester.tap(find.text('Load remote images'));
+        // Settle the snack bar enter animation and the setState rebuild
+        // that swaps in the image-loading WebView.
+        await tester.pump();
+        await tester.pump(const Duration(milliseconds: 500));
+
+        // Snack bar must be visible.
+        expect(
+          find.text('Images will be loaded automatically for this sender.'),
+          findsOneWidget,
+        );
+
+        // After 3 seconds (the snack bar's duration) plus the reverse
+        // animation, the snack bar must be gone.
+        // Regression test for #484: SnackBar with an action defaults to
+        // persist=true, which disables auto-dismiss — explicit persist:false
+        // restores duration-based dismissal.
+        await tester.pump(const Duration(seconds: 4));
+        await tester.pumpAndSettle();
+
+        expect(
+          find.text('Images will be loaded automatically for this sender.'),
+          findsNothing,
+        );
+      },
+    );
   });
 }
 

test/widget/thread_detail_screen_test.dart

@@ -249,5 +249,59 @@ void main() {
 
       expect(find.text('Body content here'), findsOneWidget);
     });
+
+    testWidgets(
+      'Load remote images snack bar auto-dismisses after 3 seconds',
+      (tester) async {
+        final email = _threadEmail();
+        await tester.pumpWidget(
+          buildApp(
+            initialLocation: '/accounts/acc-1/mailboxes/INBOX/threads/thread-1',
+            overrides: [
+              accountRepositoryProvider.overrideWithValue(
+                FakeAccountRepository([kTestAccount]),
+              ),
+              mailboxRepositoryProvider.overrideWithValue(
+                FakeMailboxRepository(),
+              ),
+              emailRepositoryProvider.overrideWithValue(
+                FakeEmailRepository(
+                  emails: [email],
+                  emailBody: const EmailBody(
+                    emailId: 'acc-1:10',
+                    htmlBody:
+                        '<p>Hi <img src="https://example.com/x.png"/></p>',
+                    attachments: [],
+                  ),
+                ),
+              ),
+            ],
+          ),
+        );
+        await tester.pumpAndSettle();
+
+        expect(find.text('Load remote images'), findsOneWidget);
+
+        await tester.tap(find.text('Load remote images'));
+        await tester.pump();
+        await tester.pump(const Duration(milliseconds: 500));
+
+        expect(
+          find.text('Images will be loaded automatically for this sender.'),
+          findsOneWidget,
+        );
+
+        // Regression test for #484: SnackBar with an action defaults to
+        // persist=true, which disables auto-dismiss — explicit persist:false
+        // restores duration-based dismissal.
+        await tester.pump(const Duration(seconds: 4));
+        await tester.pumpAndSettle();
+
+        expect(
+          find.text('Images will be loaded automatically for this sender.'),
+          findsNothing,
+        );
+      },
+    );
   });
 }