fix(web): Improve zoom behavior in photo-viewer. (#18803)

* Fix an issue where clicking the zoom-button after having zoomed in
would not zoom completely out, but leave the image in the zoomed-in
state. The new behavior properly zoomes the image completely out after
clicking the zoom-button.
* Revert to the default setting for `wheelZoomRatio` as the previous
setting of 0.2 was borderline unusable on a trackpad. This could
probably be moved to a user setting if needed.
* Add a keyboard shortcut 'z' to toggle image zoom.

web/src/lib/stores/zoom-image.store.ts

@@ -2,4 +2,3 @@ import type { ZoomImageWheelState } from '@zoom-image/core';
 import { writable } from 'svelte/store';
 
 export const photoZoomState = writable<ZoomImageWheelState>();
-export const zoomed = writable<boolean>();