Reset loading state when navigating to a new page

[amyjko]

Currently, we load a Svelte <Page> component to render a page. However, we only load it once, and so the image loading feedback that overlays the page only appears on the first page. All subsequent navigations render immediately, causing the page to jump as the cover image loads.

The way to resolve this is to reset the loading state when the page changes, reverting to the loading feedback until new images have loaded. The component doesn't yet take anything to key off, so it would need take a chapter ID, for example.