Blur out background for landing popup

[donyunardi]

To blur out the background, perhaps we can add id to the modal content:

  showModal(
    modalDialog(
      id = "landingpopup", ## adding ID
      title = title,
      content,
      footer = buttons
    )
  )

Then, we update custom.css file and use the :has pseudo-class:

#shiny-modal:has(> .modal-dialog > .modal-content > #landingpopup) {
  backdrop-filter: blur(10px);
}

Originally posted by @donyunardi in insightsengineering/teal.modules.general#585 (comment)

We decided to close the tmg PR and move the landing_popup_module to teal.
I think we forgot to add the css part to blur out the background during the changes.

[donyunardi]

PR merged to pre-release branch.
Closing.