UI enhancement pass: Image streaming, dropdown UI fixes (#909)

- Added max image sizes and compression for frontend cached images
- Fixed various UI issues with dropdown placement and animation

tests/core/test_image_cache.py

@@ -1,8 +1,25 @@
 """Tests for targeted image cache safety and fetch fallbacks."""
 
-import requests
+from io import BytesIO
 
-from shelfmark.core.image_cache import ImageCacheService
+import requests
+from PIL import Image
+
+from shelfmark.core.image_cache import (
+    ImageCacheService,
+    build_variant_cache_id,
+    create_image_variant,
+    normalize_variant_dimension,
+    normalize_variant_format,
+)
+
+
+def _make_image_bytes(
+    *, width: int = 400, height: int = 600, image_format: str = "JPEG", color: str = "navy"
+) -> bytes:
+    buffer = BytesIO()
+    Image.new("RGB", (width, height), color=color).save(buffer, format=image_format)
+    return buffer.getvalue()
 
 
 def test_is_safe_url_rejects_invalid_ipv6_url() -> None:
@@ -20,3 +37,58 @@ def test_fetch_and_cache_returns_none_on_request_exception(tmp_path, monkeypatch
 
     assert cache.fetch_and_cache("cover-1", "https://example.com/cover.jpg") is None
     assert "cover-1" not in cache._index
+
+
+def test_create_image_variant_resizes_and_transcodes_to_webp() -> None:
+    variant = create_image_variant(
+        _make_image_bytes(),
+        width=120,
+        height=180,
+        image_format="webp",
+    )
+
+    assert variant is not None
+    variant_bytes, content_type = variant
+    assert content_type == "image/webp"
+
+    with Image.open(BytesIO(variant_bytes)) as image:
+        assert image.size == (120, 180)
+
+
+def test_create_image_variant_preserves_aspect_ratio_for_single_dimension() -> None:
+    variant = create_image_variant(
+        _make_image_bytes(),
+        width=120,
+        image_format="jpeg",
+    )
+
+    assert variant is not None
+    variant_bytes, content_type = variant
+    assert content_type == "image/jpeg"
+
+    with Image.open(BytesIO(variant_bytes)) as image:
+        assert image.size == (120, 180)
+
+
+def test_create_image_variant_returns_none_when_no_change_needed() -> None:
+    image_bytes = _make_image_bytes(image_format="WEBP")
+
+    assert create_image_variant(image_bytes, image_format="webp") is None
+
+
+def test_variant_helpers_normalize_requested_variant_values() -> None:
+    assert normalize_variant_dimension("240") == 240
+    assert normalize_variant_dimension("0") is None
+    assert normalize_variant_dimension("99999") == 1024
+    assert normalize_variant_format("jpg") == "jpeg"
+    assert normalize_variant_format("weBp") == "webp"
+    assert normalize_variant_format("gif") is None
+    assert (
+        build_variant_cache_id(
+            "cover-123",
+            width=120,
+            height=180,
+            image_format="webp",
+        )
+        == "cover-123__w120_h180_fwebp"
+    )