Full-screen cards stack below modals, dismiss stray overlays

[gadenbuie]

Fixes #1360

Summary

Full-screen cards previously sat at Bootstrap's popover z-index tier (1070), so modal dialogs — including Shiny's showModal() dialogs and progress indicators — painted behind an expanded card. Cards now stack in the offcanvas tier ($zindex-offcanvas, 1045), below modals, with the card backdrop at $zindex-offcanvas-backdrop (1040), mirroring Bootstrap's offcanvas/backdrop pairing.

• Both levels are configurable at runtime via the new --bslib-card-full-screen-z-index and --bslib-card-full-screen-backdrop-z-index CSS custom properties.
• Entering full screen dismisses open dropdowns, tooltips, and popovers whose triggers live outside the card, so they no longer paint above the expanded card. Tooltips and popovers are located via their visible overlay and its aria-describedby link to the trigger, so instances created programmatically (e.g. new bootstrap.Tooltip(el)) are covered too. The dropdown lookup matches only the menu's direct sibling toggle, so parents containing multiple dropdowns target the right instance.
• The full-screen enter button shares the card's z-index variable and stays hidden while a nested bslib-offcanvas is open or transitioning, so it no longer paints through the offcanvas backdrop.

Known behavior changes: the page offcanvas backdrop no longer dims a full-screen card, and stray in-app dropdowns are dismissed when entering full screen.

Verification

Run the issue's minimal example — full-screen the card, then open the modal, which now appears above it:

library(shiny)
library(bslib)

ui <- page_fillable(
  card(max_height = 300, full_screen = TRUE,
    card_header("Card example with fullscreen + modal"),
    card_body(
      p("Full screen this card first, then press", em("open modal")),
      actionButton("open_modal", "Open modal"),
      actionButton("show_notification", "Show notification")
    )
  )
)

server <- function(input, output, session) {
  observeEvent(input$open_modal, {
    showModal(modalDialog("Modal content.", easyClose = TRUE,
      footer = modalButton("Close")))
  })
  observeEvent(input$show_notification, {
    showNotification("This is a sample notification.", type = "message")
  })
}

shinyApp(ui, server)

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