feat: remember show images per sender (#378)

## Summary

Closes #377

- Adds a new `ImageTrustedSenders` Drift table (schema v37) that stores email addresses for which remote images are loaded automatically (per device, not per account)
- When the user taps "Load remote images", the sender's address is saved and a 3-second snackbar appears with a "Settings" hyperlink to undo the choice in preferences
- Both `EmailDetailScreen` and `ThreadDetailScreen` check the trusted senders list on open and auto-load images for known senders
- The Preferences screen gains a new "Trusted image senders" section listing all saved senders with individual remove buttons

## Test plan

- [x] `dart run build_runner build` regenerates `database.g.dart` cleanly (schema v37)
- [x] `flutter analyze` — no issues
- [x] Migration test updated: checks `image_trusted_senders` table exists after upgrade and fresh install
- [x] `FakeUserPreferencesRepository` updated with three new interface methods
- [x] All 490 unit + widget tests pass (1 pre-existing golden test failure unrelated to this change)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/378

lib/core/db_schema_version.dart

@@ -1 +1 @@
-const int dbSchemaVersion = 36;
+const int dbSchemaVersion = 37;

lib/core/repositories/user_preferences_repository.dart

@@ -5,4 +5,8 @@ abstract class UserPreferencesRepository {
   Future<void> updateMenuPosition(MenuPosition position);
   Future<void> updateMailViewButtonPosition(MenuPosition position);
   Future<void> updateAfterMailViewAction(AfterMailViewAction action);
+
+  Stream<List<String>> observeTrustedImageSenders();
+  Future<void> addTrustedImageSender(String senderEmail);
+  Future<void> removeTrustedImageSender(String senderEmail);
 }