Enhancement: WP super cache - add "Preload Now" button instead of "Cancel" then enable again

### Impacted plugin

Super Cache

### What

If preload is already active, I can only cancel and re-enable to force preload. It's not helpful if I manually deleted the cache. Add a "Preload now" button even if preload timer is enabled

### How

**☝️ That's the button I'm looking for after I manually deleted my cache**

Start by locating the Super Cache preload controls and the state that currently exposes Cancel; verify behavior when preload is enabled and the cache has been manually deleted. Done means a visible Preload now action is available while the timer remains enabled and triggers a fresh preload.

Written by the indexing model from the issue text.