Refactor `ImageDisplayControl.ts`

The main file `ImageDisplayControl.ts` has grown a lot recently and should be refactored. Some of the logic (calculations, region overlays, ...) should be moved away into different files with different concerns.

Start by reading ImageDisplayControl.ts and mapping its current responsibilities, especially the calculations and region-overlay logic mentioned in the issue. Identify coherent concern boundaries before deciding where the logic belongs; done means the main file is decomposed without changing the existing image display behavior.

Written by the indexing model from the issue text.