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/data/db/database.dart

@@ -307,6 +307,17 @@ class LocalSieveApplied extends Table {
   Set<Column> get primaryKey => {accountId, messageId};
 }
 
+/// Senders for whom remote images are loaded automatically.
+/// Per-device/per-user — not tied to any email account.
+@DataClassName('ImageTrustedSenderRow')
+class ImageTrustedSenders extends Table {
+  TextColumn get senderEmail => text()();
+  DateTimeColumn get addedAt => dateTime()();
+
+  @override
+  Set<Column> get primaryKey => {senderEmail};
+}
+
 /// App-wide user preferences, stored as a singleton row (id always 1).
 @DataClassName('UserPreferencesRow')
 class UserPreferences extends Table {
@@ -345,6 +356,7 @@ class UserPreferences extends Table {
     LocalSieveApplied,
     ShareKeys,
     UserPreferences,
+    ImageTrustedSenders,
   ],
 )
 class AppDatabase extends _$AppDatabase {
@@ -611,6 +623,9 @@ class AppDatabase extends _$AppDatabase {
               userPreferences.afterMailViewAction,
             );
           }
+          if (from < 37) {
+            await m.createTable(imageTrustedSenders);
+          }
         },
       );
 }

lib/data/repositories/user_preferences_repository_impl.dart

@@ -50,6 +50,31 @@ class UserPreferencesRepositoryImpl implements UserPreferencesRepository {
         );
   }
 
+  @override
+  Stream<List<String>> observeTrustedImageSenders() {
+    return (_db.select(_db.imageTrustedSenders)
+          ..orderBy([(t) => OrderingTerm.desc(t.addedAt)]))
+        .watch()
+        .map((rows) => rows.map((r) => r.senderEmail).toList());
+  }
+
+  @override
+  Future<void> addTrustedImageSender(String senderEmail) async {
+    await _db.into(_db.imageTrustedSenders).insertOnConflictUpdate(
+          ImageTrustedSendersCompanion(
+            senderEmail: Value(senderEmail.toLowerCase()),
+            addedAt: Value(DateTime.now()),
+          ),
+        );
+  }
+
+  @override
+  Future<void> removeTrustedImageSender(String senderEmail) async {
+    await (_db.delete(_db.imageTrustedSenders)
+          ..where((t) => t.senderEmail.equals(senderEmail.toLowerCase())))
+        .go();
+  }
+
   static pref.UserPreferences _rowToModel(UserPreferencesRow? row) {
     if (row == null) return const pref.UserPreferences();
     return pref.UserPreferences(