fix(shiny-preset): Add border and styles to page main area only

[gadenbuie]

page_navbar() shiny preset dashboard styles adds border-top to the main area, but the selector matches any div directly below the page container.

bslib/inst/builtin/bs5/shiny/_rules.scss

Lines 135 to 144 in 33d135d

	.bslib-page-navbar {
	> .navbar {
	--bslib-navbar-default-bg: transparent;
	--bslib-navbar-inverse-bg: var(--#{$prefix}body-color);
	}
	> div {
	// Since we're using a transparent navbar, we need to (generally) add a border-top
	border-top: $card-border-width solid $card-border-color;

This ends up matching the shiny notification center div that's added when a notification is shown.

I've updated the selector to use .navbar + div, which more specifically targets the page content area.

While here, I also modified the themer app so that you can use a query string ?notification-duration=0 to disable notifications, or you can use any numeric value to pick a non-default duration. This is helpful for testing, otherwise the notifications are hard to pin down for styling.

[cpsievert]

Thanks!