fix: correctly notify page store subscribers

[eltigerchino]

fixes #13200

This PR ensures page store subscribers are correctly notified by creating a new object whenever we update the page store. I'm not sure why but when the same page object is used, it doesn't notify subscribers.

There was also another bug where a popstate event would cause subscribers to get notified twice because we were calling root.$set({ page }) in addition to stores.page.notify() when updating the URL.

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