Update admin theme/layout and refresh README details.

Align the project baseline with the latest admin interface styling and layout structure while documenting setup and usage updates in README.

apps/api/tests/modules/media/test_media_service.py

@@ -0,0 +1,14 @@
+import pytest
+
+from app.modules.media.service import AvatarValidationError, validate_and_process_avatar
+from tests.fixtures import make_test_png
+
+
+def test_validate_rejects_empty():
+    with pytest.raises(AvatarValidationError, match="INVALID_IMAGE"):
+        validate_and_process_avatar(b"")
+
+
+def test_validate_rejects_corrupt_bytes():
+    with pytest.raises(AvatarValidationError, match="INVALID_IMAGE"):
+        validate_and_process_avatar(b"not-an-image")