refactor: use OpenDocument picker and support PNG mime types

Author: mcbaumwolle

feature/account/avatar/impl/src/test/kotlin/net/thunderbird/feature/account/avatar/data/datasource/LocalAvatarImageDataSourceTest.kt

@@ -137,8 +137,5 @@ class LocalAvatarImageDataSourceTest {
         override suspend fun createDirectories(uri: Uri): Outcome<Unit, FileOperationError> {
             return Outcome.Success(Unit)
         }
-
-        // Add other methods if FileManager interface requires them (e.g., exists, listFiles)
-        // override suspend fun exists(uri: Uri): Boolean = true
     }
 }