## Summary
- Drop the truncated subject preview from the single-mail AppBar title; the full subject is already shown in the body header.
- Replace the popup-menu entry for **Mark as spam** with a direct `IconButton` (`Icons.report_outlined`) in the AppBar actions so the action is reachable without opening the `⋯` menu.
- Update affected widget tests for the new layout (subject is only in the body header; spam action is now a standalone button rather than a popup item).
Closes#528
## Test plan
- [x] `dart format --output=none --set-exit-if-changed lib test` — 0 changed
- [x] `dart analyze --fatal-infos lib test` — no issues
- [x] `flutter test test/widget/email_detail_screen_test.dart test/widget/email_list_screen_test.dart` — 42/42 passing
- [x] Full widget suite (`flutter test test/widget/`) — 172/172 passing
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/531
