Replace ImageManager with a new AdaptiveImageLoader that progressively
loads images through quality tiers (thumbnail → preview → original).
New components and utilities:
- AdaptiveImage: layered image renderer with thumbhash, thumbnail,
preview, and original layers with visibility managed by load state
- AdaptiveImageLoader: state machine driving the quality progression
with per-quality callbacks and error handling
- ImageLayer/Image: low-level image elements with load/error lifecycle
- PreloadManager: preloads adjacent assets for instant navigation
- AlphaBackground/DelayedLoadingSpinner: loading state UI
Zoom is handled via a derived CSS transform applied to the content
wrapper in AdaptiveImage, with the zoom library (zoomTarget: null)
only tracking state without manipulating the DOM directly.
Also adds scaleToCover to container-utils and getAssetUrls to utils.
