Don't return to fill page on refresh/new style

[aparlato]

See #62 (comment)

We currently switch back to the fill page when a style is dropped in, which also includes when the page is refreshed (which is more noticeable now that a style can be persisted).

We should remove this line and break out the code to create a background layer from the fill chart so it can run in App.svelte and always be available. This way, when a user refreshes a style, they can return to the page specified in the hash.