feat(ui): add ScrollView widget and dynamic layout on resize

Introduces a ScrollView widget with mouse wheel support and viewport clipping.
Credits and screenshot screens now use it and recompute layout on window resize.
Offset storage switched from ivec2 to vec2 with new accessors.

src/CMakeLists.txt

@@ -108,4 +108,5 @@ target_sources(${PROJECT_NAME}
         gameplay/block_manager.cpp
         ui/screenshot_ui.cpp
         scene/screenshot_scene.cpp
+        ui/scroll_view.cpp
 )