page_navbar() pages are missing banner and main landmarks

Problem

Pages built with page_navbar() (and thus shiny::navbarPage()) don't expose the HTML landmark regions that screen reader users rely on for page navigation:

• The top navigation bar is the page's visual header, but it is only a bare <nav> — there is no <header> (banner) landmark.
• When no sidebar is provided, the content area renders as a plain <div class="container-fluid"> — there is no <main> landmark. (Pages with a sidebar do get one, via page_main_container().)

What a screen reader landmark rotor shows today

For page_navbar(title = "My App", nav_panel("One", ...)):

• navigation — and nothing else. No banner, no main, so there is no landmark route to the page's content.

Expected

• banner: <header> wrapping the navbar
• navigation: <nav> (already present)
• main: <main> for the content area

Note: any fix should live in page_navbar() rather than the shared navs_bar_()/navbarPage_(), since navset_bar() can be embedded anywhere on a page, where page-level landmarks would be inappropriate (only one visible <main> per page, and a banner must not be nested in sectioning content).

Start at page_navbar() and compare its no-sidebar and sidebar content paths, while checking how navs_bar_() and navbarPage_() are used without moving page-level semantics there. Done means page_navbar() exposes header, navigation, and main landmarks, while embedded navset_bar() behavior remains appropriate.