Micrio State management

Manages the application state using Svelte stores, allowing reactive updates throughout the UI. Replaces the imperative API of Micrio 3.x.

Consists of two main state controllers:

1. State.Main: Global state for the <micr-io> element (active tour, marker, UI state).
2. State.Image: State specific to individual MicrioImage instances (view, active marker within that image).